Windows DPI scaling/highdpi support

Adds hint "SDL_WINDOWS_DPI_SCALING" which can be set to "1" to
change the SDL coordinate system units to be DPI-scaled points, rather
than pixels everywhere.

This means windows will be appropriately sized, even when created on
high-DPI displays with scaling.

e.g. requesting a 640x480 window from SDL, on a display with 125%
scaling in Windows display settings, will create a window with an
800x600 client area (in pixels).

Setting this to "1" implicitly requests process DPI awareness
(setting SDL_WINDOWS_DPI_AWARENESS is unnecessary),
and forces SDL_WINDOW_ALLOW_HIGHDPI on all windows.

include/SDL_hints.h

@@ -1838,6 +1838,27 @@ extern "C" {
  */
 #define SDL_HINT_WINDOWS_DPI_AWARENESS "SDL_WINDOWS_DPI_AWARENESS"
 
+ /**
+ * \brief Uses DPI-scaled points as the SDL coordinate system on Windows.
+ * 
+ *  This changes the SDL coordinate system units to be DPI-scaled points, rather than pixels everywhere.
+ *  This means windows will be appropriately sized, even when created on high-DPI displays with scaling.
+ * 
+ *  e.g. requesting a 640x480 window from SDL, on a display with 125% scaling in Windows display settings,
+ *  will create a window with an 800x600 client area (in pixels).
+ *
+ *  Setting this to "1" implicitly requests process DPI awareness (setting SDL_WINDOWS_DPI_AWARENESS is unnecessary),
+ *  and forces SDL_WINDOW_ALLOW_HIGHDPI on all windows.
+ * 
+ *  This variable can be set to the following values:
+ *    "0"       - SDL coordinates equal Windows coordinates. No automatic window resizing when dragging
+ *                between monitors with different scale factors (unless this is performed by
+ *                Windows itself, which is the case when the process is DPI unaware).
+ *    "1"       - SDL coordinates are in DPI-scaled points. Automatically resize windows as needed on
+ *                displays with non-100% scale factors.
+ */
+#define SDL_HINT_WINDOWS_DPI_SCALING "SDL_WINDOWS_DPI_SCALING"
+
 /**
  *  \brief  A variable controlling whether the window frame and title bar are interactive when the cursor is hidden 
  *

src/render/direct3d/SDL_render_d3d.c

@@ -308,7 +308,7 @@ D3D_ActivateRenderer(SDL_Renderer * renderer)
         int w, h;
         Uint32 window_flags = SDL_GetWindowFlags(window);
 
-        SDL_GetWindowSize(window, &w, &h);
+        WIN_GetDrawableSize(window, &w, &h);
         data->pparams.BackBufferWidth = w;
         data->pparams.BackBufferHeight = h;
         if (window_flags & SDL_WINDOW_FULLSCREEN && (window_flags & SDL_WINDOW_FULLSCREEN_DESKTOP) != SDL_WINDOW_FULLSCREEN_DESKTOP) {
@@ -354,6 +354,13 @@ D3D_WindowEvent(SDL_Renderer * renderer, const SDL_WindowEvent *event)
     }
 }
 
+static int
+D3D_GetOutputSize(SDL_Renderer * renderer, int *w, int *h)
+{
+    WIN_GetDrawableSize(renderer->window, w, h);
+    return 0;
+}
+
 static D3DBLEND
 GetBlendFunc(SDL_BlendFactor factor)
 {
@@ -1616,6 +1623,7 @@ D3D_CreateRenderer(SDL_Window * window, Uint32 flags)
     }
 
     renderer->WindowEvent = D3D_WindowEvent;
+    renderer->GetOutputSize = D3D_GetOutputSize;
     renderer->SupportsBlendMode = D3D_SupportsBlendMode;
     renderer->CreateTexture = D3D_CreateTexture;
     renderer->UpdateTexture = D3D_UpdateTexture;
@@ -1645,7 +1653,7 @@ D3D_CreateRenderer(SDL_Window * window, Uint32 flags)
     SDL_GetWindowWMInfo(window, &windowinfo);
 
     window_flags = SDL_GetWindowFlags(window);
-    SDL_GetWindowSize(window, &w, &h);
+    WIN_GetDrawableSize(window, &w, &h);
     SDL_GetWindowDisplayMode(window, &fullscreen_mode);
 
     SDL_zero(pparams);

src/render/direct3d11/SDL_render_d3d11.c

@@ -27,6 +27,7 @@
 
 #define COBJMACROS
 #include "../../core/windows/SDL_windows.h"
+#include "../../video/windows/SDL_windowswindow.h"
 #include "SDL_hints.h"
 #include "SDL_loadso.h"
 #include "SDL_syswm.h"
@@ -910,7 +911,7 @@ D3D11_CreateWindowSizeDependentResources(SDL_Renderer * renderer)
     /* The width and height of the swap chain must be based on the display's
      * non-rotated size.
      */
-    SDL_GetWindowSize(renderer->window, &w, &h);
+    WIN_GetDrawableSize(renderer->window, &w, &h);
     data->rotation = D3D11_GetCurrentRotation();
     /* SDL_Log("%s: windowSize={%d,%d}, orientation=%d\n", __FUNCTION__, w, h, (int)data->rotation); */
     if (D3D11_IsDisplayRotated90Degrees(data->rotation)) {
@@ -1051,6 +1052,13 @@ D3D11_WindowEvent(SDL_Renderer * renderer, const SDL_WindowEvent *event)
     }
 }
 
+static int
+D3D11_GetOutputSize(SDL_Renderer * renderer, int *w, int *h)
+{
+    WIN_GetDrawableSize(renderer->window, w, h);
+    return 0;
+}
+
 static SDL_bool
 D3D11_SupportsBlendMode(SDL_Renderer * renderer, SDL_BlendMode blendMode)
 {
@@ -2366,6 +2374,7 @@ D3D11_CreateRenderer(SDL_Window * window, Uint32 flags)
     data->identity = MatrixIdentity();
 
     renderer->WindowEvent = D3D11_WindowEvent;
+    renderer->GetOutputSize = D3D11_GetOutputSize;
     renderer->SupportsBlendMode = D3D11_SupportsBlendMode;
     renderer->CreateTexture = D3D11_CreateTexture;
     renderer->UpdateTexture = D3D11_UpdateTexture;

src/video/SDL_video.c

@@ -1374,12 +1374,17 @@ SDL_UpdateFullscreenMode(SDL_Window * window, SDL_bool fullscreen)
 
                 /* Generate a mode change event here */
                 if (resized) {
-#ifndef ANDROID
+#if !defined(ANDROID) && !defined(WIN32)
                     /* Android may not resize the window to exactly what our fullscreen mode is, especially on
                      * windowed Android environments like the Chromebook or Samsung DeX.  Given this, we shouldn't
                      * use fullscreen_mode.w and fullscreen_mode.h, but rather get our current native size.  As such,
                      * Android's SetWindowFullscreen will generate the window event for us with the proper final size.
                      */
+
+                    /* This is also unnecessary on Win32 (WIN_SetWindowFullscreen calls SetWindowPos,
+                     * WM_WINDOWPOSCHANGED will send SDL_WINDOWEVENT_RESIZED). Also, on Windows with DPI scaling enabled,
+                     * we're keeping modes in pixels, but window sizes in dpi-scaled points, so this would be a unit mismatch.
+                     */
                     SDL_SendWindowEvent(other, SDL_WINDOWEVENT_RESIZED,
                                         fullscreen_mode.w, fullscreen_mode.h);
 #endif

src/video/windows/SDL_windowsevents.c

@@ -749,7 +749,12 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
                 /* Only generate mouse events for real mouse */
                 if (GetMouseMessageSource() != SDL_MOUSE_EVENT_SOURCE_TOUCH &&
                     lParam != data->last_pointer_update) {
-                    SDL_SendMouseMotion(data->window, 0, 0, GET_X_LPARAM(lParam), GET_Y_LPARAM(lParam));
+                    int x = GET_X_LPARAM(lParam);
+                    int y = GET_Y_LPARAM(lParam);
+
+                    WIN_ClientPointToSDL(data->window, &x, &y);
+
+                    SDL_SendMouseMotion(data->window, 0, 0, x, y);
                 }
             }
         }
@@ -1068,6 +1073,12 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
             SDL_GetWindowMinimumSize(data->window, &min_w, &min_h);
             SDL_GetWindowMaximumSize(data->window, &max_w, &max_h);
 
+            /* Convert w, h, min_w, min_h, max_w, max_h from dpi-scaled points to pixels,
+               treating them as coordinates within the client area. */
+            WIN_ClientPointFromSDL(data->window, &w, &h);
+            WIN_ClientPointFromSDL(data->window, &min_w, &min_h);
+            WIN_ClientPointFromSDL(data->window, &max_w, &max_h);
+
             /* Store in min_w and min_h difference between current size and minimal
                size so we don't need to call AdjustWindowRectEx twice */
             min_w -= w;
@@ -1168,16 +1179,21 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
 
             x = rect.left;
             y = rect.top;
+            WIN_ScreenPointToSDL(&x, &y);
+
             SDL_SendWindowEvent(data->window, SDL_WINDOWEVENT_MOVED, x, y);
 
+            /* Convert client area width/height from pixels to dpi-scaled points */
             w = rect.right - rect.left;
             h = rect.bottom - rect.top;
+            WIN_ClientPointToSDL(data->window, &w, &h);
+
             SDL_SendWindowEvent(data->window, SDL_WINDOWEVENT_RESIZED, w, h);
 
 #ifdef HIGHDPI_DEBUG
-            SDL_Log("WM_WINDOWPOSCHANGED: Windows client rect (pixels): (%d, %d) (%d x %d)\tSDL client rect: (%d, %d) (%d x %d)\tGetDpiForWindow: %d",
+            SDL_Log("WM_WINDOWPOSCHANGED: Windows client rect (pixels): (%d, %d) (%d x %d)\tSDL client rect (points): (%d, %d) (%d x %d) cached dpi %d, windows reported dpi %d",
-                    rect.left, rect.top, rect.right - rect.left, rect.bottom - rect.top,
+                rect.left, rect.top, rect.right - rect.left, rect.bottom - rect.top,
-                    x, y, w, h, data->videodata->GetDpiForWindow ? (int)data->videodata->GetDpiForWindow(data->hwnd) : 0);
+                x, y, w, h, data->scaling_dpi, data->videodata->GetDpiForWindow ? data->videodata->GetDpiForWindow(data->hwnd) : 0);
 #endif
 
             /* Forces a WM_PAINT event */
@@ -1385,6 +1401,7 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
                     NCCALCSIZE_PARAMS *params = (NCCALCSIZE_PARAMS *)lParam;
                     w = data->window->windowed.w;
                     h = data->window->windowed.h;
+                    WIN_ClientPointFromSDL(data->window, &w, &h);
                     params->rgrc[0].right = params->rgrc[0].left + w;
                     params->rgrc[0].bottom = params->rgrc[0].top + h;
                 }
@@ -1405,6 +1422,7 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
                     SDL_HitTestResult rc;
                     point.x = winpoint.x;
                     point.y = winpoint.y;
+                    WIN_ClientPointToSDL(data->window, &point.x, &point.y);
                     rc = window->hit_test(window, &point, window->hit_test_data);
                     switch (rc) {
                         #define POST_HIT_TEST(ret) { SDL_SendWindowEvent(data->window, SDL_WINDOWEVENT_HIT_TEST, 0, 0); return ret; }
@@ -1474,6 +1492,14 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
                 query_client_h_win = sizeInOut->cy - frame_h;
             }
 
+            /* Convert to new dpi if we are using scaling. 
+             * Otherwise leave as pixels.
+             */
+            if (data->videodata->dpi_scaling_enabled) {
+                query_client_w_win = MulDiv(query_client_w_win, nextDPI, prevDPI);
+                query_client_h_win = MulDiv(query_client_h_win, nextDPI, prevDPI);
+            }
+
             /* Add the window frame size that would be used at nextDPI */
             {
                 RECT rect = {0};
@@ -1508,6 +1534,15 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
                 suggestedRect->left, suggestedRect->top, suggestedRect->right - suggestedRect->left, suggestedRect->bottom - suggestedRect->top);
 #endif
 
+            if (data->videodata->dpi_scaling_enabled) {
+                /* Update the cached DPI value for this window */
+                data->scaling_dpi = newDPI;
+
+                /* Send a SDL_WINDOWEVENT_SIZE_CHANGED saying that the client size (in dpi-scaled points) is unchanged. 
+                   Renderers need to get this to know that the framebuffer size changed. */
+                SDL_SendWindowEvent(data->window, SDL_WINDOWEVENT_SIZE_CHANGED, data->window->w, data->window->h);
+            }
+
             if (data->expected_resize) {
                 /* This DPI change is coming from an explicit SetWindowPos call within SDL.
                    Assume all call sites are calculating the DPI-aware frame correctly, so
@@ -1539,6 +1574,12 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
                 rect.right = data->window->w;
                 rect.bottom = data->window->h;
 
+                if (data->videodata->dpi_scaling_enabled) {
+                    /* scale client size to from points to the new DPI */
+                    rect.right = MulDiv(rect.right, newDPI, 96);
+                    rect.bottom = MulDiv(rect.bottom, newDPI, 96);
+                }
+
                 if (!(data->window->flags & SDL_WINDOW_BORDERLESS)) {
                     AdjustWindowRectEx(&rect, style, menu, 0);
                 }

src/video/windows/SDL_windowsmodes.c

@@ -31,6 +31,7 @@
 #endif
 
 /* #define DEBUG_MODES */
+/* #define HIGHDPI_DEBUG_VERBOSE */
 
 static void
 WIN_UpdateDisplayMode(_THIS, LPCWSTR deviceName, DWORD index, SDL_DisplayMode * mode)
@@ -50,6 +51,15 @@ WIN_UpdateDisplayMode(_THIS, LPCWSTR deviceName, DWORD index, SDL_DisplayMode *
         int logical_width = GetDeviceCaps( hdc, HORZRES );
         int logical_height = GetDeviceCaps( hdc, VERTRES );
 
+        /* High-DPI notes:
+
+           If DPI-unaware:
+             - GetDeviceCaps( hdc, HORZRES ) will return the monitor width in points.
+             - DeviceMode.dmPelsWidth is actual pixels (unlike almost all other Windows API's,
+               it's not virtualized when DPI unaware).
+
+           If DPI-aware:
+            - GetDeviceCaps( hdc, HORZRES ) will return pixels, same as DeviceMode.dmPelsWidth */
         mode->w = logical_width;
         mode->h = logical_height;
         
@@ -301,10 +311,46 @@ WIN_InitModes(_THIS)
     return 0;
 }
 
+/**
+ * Convert the monitor rect and work rect from pixels to the SDL coordinate system (monitor origins are in pixels, 
+ * monitor size in DPI-scaled points).
+ * 
+ * No-op if DPI scaling is not enabled.
+ */
+static void
+WIN_MonitorInfoToSDL(const SDL_VideoData *videodata, HMONITOR monitor, MONITORINFO *info)
+{
+    UINT xdpi, ydpi;
+
+    if (!videodata->dpi_scaling_enabled) {
+        return;
+    }
+
+    /* Check for Windows < 8.1*/
+    if (!videodata->GetDpiForMonitor) {
+        return;
+    }
+    if (videodata->GetDpiForMonitor(monitor, MDT_EFFECTIVE_DPI, &xdpi, &ydpi) != S_OK) {
+        /* Shouldn't happen? */
+        return;
+    }
+
+    /* Convert monitor size to points, leaving the monitor position in pixels */
+    info->rcMonitor.right = info->rcMonitor.left + MulDiv(info->rcMonitor.right - info->rcMonitor.left, 96, xdpi);
+    info->rcMonitor.bottom = info->rcMonitor.top + MulDiv(info->rcMonitor.bottom - info->rcMonitor.top, 96, ydpi);
+
+    /* Convert monitor work rect to points */
+    info->rcWork.left   = info->rcMonitor.left + MulDiv(info->rcWork.left - info->rcMonitor.left,  96, xdpi);
+    info->rcWork.right  = info->rcMonitor.left + MulDiv(info->rcWork.right - info->rcMonitor.left, 96, xdpi);
+    info->rcWork.top    = info->rcMonitor.top  + MulDiv(info->rcWork.top - info->rcMonitor.top,    96, ydpi);
+    info->rcWork.bottom = info->rcMonitor.top  + MulDiv(info->rcWork.bottom - info->rcMonitor.top, 96, ydpi);
+}
+
 int
 WIN_GetDisplayBounds(_THIS, SDL_VideoDisplay * display, SDL_Rect * rect)
 {
     const SDL_DisplayData *data = (const SDL_DisplayData *)display->driverdata;
+    const SDL_VideoData *videodata = (SDL_VideoData *)display->device->driverdata;
     MONITORINFO minfo;
     BOOL rc;
 
@@ -316,6 +362,7 @@ WIN_GetDisplayBounds(_THIS, SDL_VideoDisplay * display, SDL_Rect * rect)
         return SDL_SetError("Couldn't find monitor data");
     }
 
+    WIN_MonitorInfoToSDL(videodata, data->MonitorHandle, &minfo);
     rect->x = minfo.rcMonitor.left;
     rect->y = minfo.rcMonitor.top;
     rect->w = minfo.rcMonitor.right - minfo.rcMonitor.left;
@@ -387,6 +434,7 @@ int
 WIN_GetDisplayUsableBounds(_THIS, SDL_VideoDisplay * display, SDL_Rect * rect)
 {
     const SDL_DisplayData *data = (const SDL_DisplayData *)display->driverdata;
+    const SDL_VideoData *videodata = (SDL_VideoData *)display->device->driverdata;
     MONITORINFO minfo;
     BOOL rc;
 
@@ -398,6 +446,7 @@ WIN_GetDisplayUsableBounds(_THIS, SDL_VideoDisplay * display, SDL_Rect * rect)
         return SDL_SetError("Couldn't find monitor data");
     }
 
+    WIN_MonitorInfoToSDL(videodata, data->MonitorHandle, &minfo);
     rect->x = minfo.rcWork.left;
     rect->y = minfo.rcWork.top;
     rect->w = minfo.rcWork.right - minfo.rcWork.left;
@@ -406,6 +455,190 @@ WIN_GetDisplayUsableBounds(_THIS, SDL_VideoDisplay * display, SDL_Rect * rect)
     return 0;
 }
 
+/**
+ * If x, y are outside of rect, snaps them to the closest point inside rect
+ * (between rect->x, rect->y, inclusive, and rect->x + w, rect->y + h, exclusive)
+ */
+static void
+SDL_GetClosestPointOnRect(const SDL_Rect *rect, int *x, int *y)
+{
+    const int right = rect->x + rect->w - 1;
+    const int bottom = rect->y + rect->h - 1;
+
+    if (*x < rect->x) {
+        *x = rect->x;
+    } else if (*x > right) {
+        *x = right;
+    }
+
+    if (*y < rect->y) {
+        *y = rect->y;
+    } else if (*y > bottom) {
+        *y = bottom;
+    }
+}
+
+/**
+ * Returns the display index of the display which either encloses the given point
+ * or is closest to it. The point is in SDL screen coordinates.
+ */
+static int
+SDL_GetPointDisplayIndex(int x, int y)
+{
+    int i, dist;
+    int closest = -1;
+    int closest_dist = 0x7FFFFFFF;
+    SDL_Point closest_point_on_display;
+    SDL_Point delta;
+    SDL_Rect rect;
+    SDL_VideoDevice *_this = SDL_GetVideoDevice();
+    SDL_Point point;
+    point.x = x;
+    point.y = y;
+
+    for (i = 0; i < _this->num_displays; ++i) {
+        /* Check for an exact match */
+        SDL_GetDisplayBounds(i, &rect);
+        if (SDL_EnclosePoints(&point, 1, &rect, NULL)) {
+            return i;
+        }
+
+        /* Snap x, y to the display rect */
+        closest_point_on_display = point;
+        SDL_GetClosestPointOnRect(&rect, &closest_point_on_display.x, &closest_point_on_display.y);
+
+        delta.x = point.x - closest_point_on_display.x;
+        delta.y = point.y - closest_point_on_display.y;
+        dist = (delta.x*delta.x + delta.y*delta.y);
+        if (dist < closest_dist) {
+            closest = i;
+            closest_dist = dist;
+        }
+    }
+    if (closest < 0) {
+        SDL_SetError("Couldn't find any displays");
+    }
+    return closest;
+}
+
+/**
+ * Convert a point from the SDL coordinate system (monitor origins are in pixels, 
+ * offset within a monitor in DPI-scaled points) to Windows virtual screen coordinates (pixels).
+ * 
+ * No-op if DPI scaling is not enabled (returns 96 dpi).
+ * 
+ * Returns the DPI of the monitor that was closest to x, y and used for the conversion.
+ */
+void WIN_ScreenPointFromSDL(int *x, int *y, int *dpiOut)
+{
+    const SDL_VideoDevice *videodevice = SDL_GetVideoDevice();
+    const SDL_VideoData *videodata;
+    int displayIndex;
+    SDL_Rect bounds;
+    float ddpi, hdpi, vdpi;
+    int x_sdl, y_sdl;
+
+    if (dpiOut) {
+        *dpiOut = 96;
+    }
+
+    if (!videodevice || !videodevice->driverdata) {
+        return;
+    }
+
+    videodata = (SDL_VideoData *)videodevice->driverdata;
+    if (!videodata->dpi_scaling_enabled) {
+        return;
+    }
+
+    /* Can't use MonitorFromPoint for this because we currently have SDL coordinates, not pixels */
+    displayIndex = SDL_GetPointDisplayIndex(*x, *y);
+
+    if (displayIndex < 0) {
+        return;
+    }
+
+    if (SDL_GetDisplayBounds(displayIndex, &bounds) < 0
+        || SDL_GetDisplayDPI(displayIndex, &ddpi, &hdpi, &vdpi) < 0) {
+        return;
+    }
+
+    if (dpiOut) {
+        *dpiOut = (int) ddpi;
+    }
+
+    /* Undo the DPI-scaling within the monitor bounds to convert back to pixels */
+    x_sdl = *x;
+    y_sdl = *y;
+    *x = bounds.x + MulDiv(x_sdl - bounds.x, (int)ddpi, 96);
+    *y = bounds.y + MulDiv(y_sdl - bounds.y, (int)ddpi, 96);
+
+#ifdef HIGHDPI_DEBUG_VERBOSE
+    SDL_Log("WIN_ScreenPointFromSDL: (%d, %d) points -> (%d x %d) pixels, using %d DPI monitor",
+        x_sdl, y_sdl, *x, *y, (int)ddpi);
+#endif
+}
+
+/**
+ * Convert a point from Windows virtual screen coordinates (pixels) to the SDL
+ * coordinate system (monitor origins are in pixels, offset within a monitor in DPI-scaled points).
+ *
+ * No-op if DPI scaling is not enabled.
+ */
+void WIN_ScreenPointToSDL(int *x, int *y)
+{
+    const SDL_VideoDevice *videodevice = SDL_GetVideoDevice();
+    const SDL_VideoData *videodata;
+    POINT point;
+    HMONITOR monitor;
+    int i, displayIndex;
+    SDL_Rect bounds;
+    float ddpi, hdpi, vdpi;
+    int x_pixels, y_pixels;
+
+    if (!videodevice || !videodevice->driverdata) {
+        return;
+    }
+
+    videodata = (SDL_VideoData *)videodevice->driverdata;
+    if (!videodata->dpi_scaling_enabled) {
+        return;
+    }
+    
+    point.x = *x;
+    point.y = *y;
+    monitor = MonitorFromPoint(point, MONITOR_DEFAULTTONEAREST);
+
+    /* Search for the corresponding SDL monitor */
+    displayIndex = -1;
+    for (i = 0; i < videodevice->num_displays; ++i) {
+        SDL_DisplayData *driverdata = (SDL_DisplayData *)videodevice->displays[i].driverdata;
+        if (driverdata->MonitorHandle == monitor) {
+            displayIndex = i;
+        }
+    }
+    if (displayIndex == -1) {
+        return;
+    }
+
+    /* Get SDL display properties */
+    if (SDL_GetDisplayBounds(displayIndex, &bounds) < 0
+        || SDL_GetDisplayDPI(displayIndex, &ddpi, &hdpi, &vdpi) < 0) {
+        return;
+    }
+
+    /* Convert the point's offset within the monitor from pixels to DPI-scaled points */
+    x_pixels = *x;
+    y_pixels = *y;
+    *x = bounds.x + MulDiv(x_pixels - bounds.x, 96, (int)ddpi);
+    *y = bounds.y + MulDiv(y_pixels - bounds.y, 96, (int)ddpi);
+
+#ifdef HIGHDPI_DEBUG_VERBOSE
+    SDL_Log("WIN_ScreenPointToSDL: (%d, %d) pixels -> (%d x %d) points, using %d DPI monitor",
+        x_pixels, y_pixels, *x, *y, (int)ddpi);
+#endif
+}
+
 void
 WIN_GetDisplayModes(_THIS, SDL_VideoDisplay * display)
 {
@@ -432,6 +665,37 @@ WIN_GetDisplayModes(_THIS, SDL_VideoDisplay * display)
     }
 }
 
+#ifdef DEBUG_MODES
+static void
+WIN_LogMonitor(_THIS, HMONITOR mon)
+{
+    const SDL_VideoData *vid_data = (const SDL_VideoData *)_this->driverdata;
+    MONITORINFOEX minfo;
+    UINT xdpi = 0, ydpi = 0;
+    char *name_utf8;
+
+    if (vid_data->GetDpiForMonitor) {
+        vid_data->GetDpiForMonitor(mon, MDT_EFFECTIVE_DPI, &xdpi, &ydpi);
+    }
+
+    SDL_zero(minfo);
+    minfo.cbSize = sizeof(minfo);
+    GetMonitorInfo(mon, (LPMONITORINFO)&minfo);
+
+    name_utf8 = WIN_StringToUTF8(minfo.szDevice);
+
+    SDL_Log("WIN_LogMonitor: monitor \"%s\": dpi: %d windows screen coordinates: %d, %d, %dx%d",
+        name_utf8,
+        xdpi,
+        minfo.rcMonitor.left,
+        minfo.rcMonitor.top,
+        minfo.rcMonitor.right - minfo.rcMonitor.left,
+        minfo.rcMonitor.bottom - minfo.rcMonitor.top);
+
+    SDL_free(name_utf8);
+}
+#endif
+
 int
 WIN_SetDisplayMode(_THIS, SDL_VideoDisplay * display, SDL_DisplayMode * mode)
 {
@@ -439,9 +703,30 @@ WIN_SetDisplayMode(_THIS, SDL_VideoDisplay * display, SDL_DisplayMode * mode)
     SDL_DisplayModeData *data = (SDL_DisplayModeData *) mode->driverdata;
     LONG status;
 
+#ifdef DEBUG_MODES
+    SDL_Log("WIN_SetDisplayMode: monitor state before mode change:");
+    WIN_LogMonitor(_this, displaydata->MonitorHandle);
+#endif
+
+    /* High-DPI notes:
+
+       - ChangeDisplaySettingsEx always takes pixels.
+       - e.g. if the display is set to 2880x1800 with 200% scaling in Display Settings
+         - calling ChangeDisplaySettingsEx with a dmPelsWidth/Height other than 2880x1800 will
+           change the monitor DPI to 96. (100% scaling)
+         - calling ChangeDisplaySettingsEx with a dmPelsWidth/Height of 2880x1800 (or a NULL DEVMODE*) will
+           reset the monitor DPI to 192. (200% scaling)
+
+       NOTE: these are temporary changes in DPI, not modifications to the Control Panel setting. */
     if (mode->driverdata == display->desktop_mode.driverdata) {
+#ifdef DEBUG_MODES
+        SDL_Log("WIN_SetDisplayMode: resetting to original resolution");
+#endif
         status = ChangeDisplaySettingsExW(displaydata->DeviceName, NULL, NULL, CDS_FULLSCREEN, NULL);
     } else {
+#ifdef DEBUG_MODES
+        SDL_Log("WIN_SetDisplayMode: changing to %dx%d pixels", data->DeviceMode.dmPelsWidth, data->DeviceMode.dmPelsHeight);
+#endif
         status = ChangeDisplaySettingsExW(displaydata->DeviceName, &data->DeviceMode, NULL, CDS_FULLSCREEN, NULL);
     }
     if (status != DISP_CHANGE_SUCCESSFUL) {
@@ -462,6 +747,12 @@ WIN_SetDisplayMode(_THIS, SDL_VideoDisplay * display, SDL_DisplayMode * mode)
         }
         return SDL_SetError("ChangeDisplaySettingsEx() failed: %s", reason);
     }
+
+#ifdef DEBUG_MODES
+    SDL_Log("WIN_SetDisplayMode: monitor state after mode change:");
+    WIN_LogMonitor(_this, displaydata->MonitorHandle);
+#endif
+
     EnumDisplaySettingsW(displaydata->DeviceName, ENUM_CURRENT_SETTINGS, &data->DeviceMode);
     WIN_UpdateDisplayMode(_this, displaydata->DeviceName, ENUM_CURRENT_SETTINGS, mode);
     return 0;

src/video/windows/SDL_windowsmodes.h

@@ -38,6 +38,8 @@ typedef struct
 extern int WIN_InitModes(_THIS);
 extern int WIN_GetDisplayBounds(_THIS, SDL_VideoDisplay * display, SDL_Rect * rect);
 extern int WIN_GetDisplayUsableBounds(_THIS, SDL_VideoDisplay * display, SDL_Rect * rect);
+extern void WIN_ScreenPointFromSDL(int *x, int *y, int *dpiOut);
+extern void WIN_ScreenPointToSDL(int *x, int *y);
 extern int WIN_GetDisplayDPI(_THIS, SDL_VideoDisplay * display, float * ddpi, float * hdpi, float * vdpi);
 extern void WIN_GetDisplayModes(_THIS, SDL_VideoDisplay * display);
 extern int WIN_SetDisplayMode(_THIS, SDL_VideoDisplay * display, SDL_DisplayMode * mode);

src/video/windows/SDL_windowsmouse.c

@@ -290,6 +290,7 @@ WIN_WarpMouseGlobal(int x, int y)
 {
     POINT pt;
 
+    WIN_ScreenPointFromSDL(&x, &y, NULL);
     pt.x = x;
     pt.y = y;
     SetCursorPos(pt.x, pt.y);
@@ -333,6 +334,7 @@ WIN_GetGlobalMouseState(int *x, int *y)
     GetCursorPos(&pt);
     *x = (int) pt.x;
     *y = (int) pt.y;
+    WIN_ScreenPointToSDL(x, y);
 
     retval |= GetAsyncKeyState(!swapButtons ? VK_LBUTTON : VK_RBUTTON) & 0x8000 ? SDL_BUTTON_LMASK : 0;
     retval |= GetAsyncKeyState(!swapButtons ? VK_RBUTTON : VK_LBUTTON) & 0x8000 ? SDL_BUTTON_RMASK : 0;

src/video/windows/SDL_windowsopengl.c

@@ -676,6 +676,7 @@ WIN_GL_CreateContext(_THIS, SDL_Window * window)
         _this->GL_UnloadLibrary = WIN_GLES_UnloadLibrary;
         _this->GL_CreateContext = WIN_GLES_CreateContext;
         _this->GL_MakeCurrent = WIN_GLES_MakeCurrent;
+        _this->GL_GetDrawableSize = WIN_GLES_GetDrawableSize;
         _this->GL_SetSwapInterval = WIN_GLES_SetSwapInterval;
         _this->GL_GetSwapInterval = WIN_GLES_GetSwapInterval;
         _this->GL_SwapWindow = WIN_GLES_SwapWindow;
@@ -822,6 +823,12 @@ WIN_GL_MakeCurrent(_THIS, SDL_Window * window, SDL_GLContext context)
     return 0;
 }
 
+void
+WIN_GL_GetDrawableSize(_THIS, SDL_Window *window, int *w, int *h)
+{
+    WIN_GetDrawableSize(window, w, h);
+}
+
 int
 WIN_GL_SetSwapInterval(_THIS, int interval)
 {

src/video/windows/SDL_windowsopengl.h

@@ -71,6 +71,7 @@ extern int WIN_GL_SetupWindow(_THIS, SDL_Window * window);
 extern SDL_GLContext WIN_GL_CreateContext(_THIS, SDL_Window * window);
 extern int WIN_GL_MakeCurrent(_THIS, SDL_Window * window,
                               SDL_GLContext context);
+extern void WIN_GL_GetDrawableSize(_THIS, SDL_Window *window, int *w, int *h);
 extern int WIN_GL_SetSwapInterval(_THIS, int interval);
 extern int WIN_GL_GetSwapInterval(_THIS);
 extern int WIN_GL_SwapWindow(_THIS, SDL_Window * window);

src/video/windows/SDL_windowsopengles.c

@@ -25,6 +25,7 @@
 #include "SDL_windowsvideo.h"
 #include "SDL_windowsopengles.h"
 #include "SDL_windowsopengl.h"
+#include "SDL_windowswindow.h"
 
 /* EGL implementation of SDL OpenGL support */
 
@@ -40,6 +41,7 @@ WIN_GLES_LoadLibrary(_THIS, const char *path) {
         _this->GL_UnloadLibrary = WIN_GL_UnloadLibrary;
         _this->GL_CreateContext = WIN_GL_CreateContext;
         _this->GL_MakeCurrent = WIN_GL_MakeCurrent;
+        _this->GL_GetDrawableSize = WIN_GL_GetDrawableSize;
         _this->GL_SetSwapInterval = WIN_GL_SetSwapInterval;
         _this->GL_GetSwapInterval = WIN_GL_GetSwapInterval;
         _this->GL_SwapWindow = WIN_GL_SwapWindow;
@@ -72,6 +74,7 @@ WIN_GLES_CreateContext(_THIS, SDL_Window * window)
         _this->GL_UnloadLibrary = WIN_GL_UnloadLibrary;
         _this->GL_CreateContext = WIN_GL_CreateContext;
         _this->GL_MakeCurrent = WIN_GL_MakeCurrent;
+        _this->GL_GetDrawableSize = WIN_GL_GetDrawableSize;
         _this->GL_SetSwapInterval = WIN_GL_SetSwapInterval;
         _this->GL_GetSwapInterval = WIN_GL_GetSwapInterval;
         _this->GL_SwapWindow = WIN_GL_SwapWindow;
@@ -99,6 +102,12 @@ WIN_GLES_DeleteContext(_THIS, SDL_GLContext context)
 SDL_EGL_SwapWindow_impl(WIN)
 SDL_EGL_MakeCurrent_impl(WIN)
 
+void
+WIN_GLES_GetDrawableSize(_THIS, SDL_Window* window, int* w, int* h)
+{
+    WIN_GetDrawableSize(window, w, h);
+}
+
 int
 WIN_GLES_SetupWindow(_THIS, SDL_Window * window)
 {

src/video/windows/SDL_windowsopengles.h

@@ -39,6 +39,7 @@ extern int WIN_GLES_LoadLibrary(_THIS, const char *path);
 extern SDL_GLContext WIN_GLES_CreateContext(_THIS, SDL_Window * window);
 extern int WIN_GLES_SwapWindow(_THIS, SDL_Window * window);
 extern int WIN_GLES_MakeCurrent(_THIS, SDL_Window * window, SDL_GLContext context);
+extern void WIN_GLES_GetDrawableSize(_THIS, SDL_Window* window, int* w, int* h);
 extern void WIN_GLES_DeleteContext(_THIS, SDL_GLContext context);
 extern int WIN_GLES_SetupWindow(_THIS, SDL_Window * window);
 

src/video/windows/SDL_windowsvideo.c

@@ -203,6 +203,7 @@ WIN_CreateDevice(int devindex)
     device->GL_UnloadLibrary = WIN_GL_UnloadLibrary;
     device->GL_CreateContext = WIN_GL_CreateContext;
     device->GL_MakeCurrent = WIN_GL_MakeCurrent;
+    device->GL_GetDrawableSize = WIN_GL_GetDrawableSize;
     device->GL_SetSwapInterval = WIN_GL_SetSwapInterval;
     device->GL_GetSwapInterval = WIN_GL_GetSwapInterval;
     device->GL_SwapWindow = WIN_GL_SwapWindow;
@@ -214,6 +215,7 @@ WIN_CreateDevice(int devindex)
     device->GL_UnloadLibrary = WIN_GLES_UnloadLibrary;
     device->GL_CreateContext = WIN_GLES_CreateContext;
     device->GL_MakeCurrent = WIN_GLES_MakeCurrent;
+    device->GL_GetDrawableSize = WIN_GLES_GetDrawableSize;
     device->GL_SetSwapInterval = WIN_GLES_SetSwapInterval;
     device->GL_GetSwapInterval = WIN_GLES_GetSwapInterval;
     device->GL_SwapWindow = WIN_GLES_SwapWindow;
@@ -224,6 +226,7 @@ WIN_CreateDevice(int devindex)
     device->Vulkan_UnloadLibrary = WIN_Vulkan_UnloadLibrary;
     device->Vulkan_GetInstanceExtensions = WIN_Vulkan_GetInstanceExtensions;
     device->Vulkan_CreateSurface = WIN_Vulkan_CreateSurface;
+    device->Vulkan_GetDrawableSize = WIN_GL_GetDrawableSize;
 #endif
 
     device->StartTextInput = WIN_StartTextInput;
@@ -373,12 +376,25 @@ WIN_InitDPIAwareness(_THIS)
     }
 }
 
+static void
+WIN_InitDPIScaling(_THIS)
+{
+    SDL_VideoData* data = (SDL_VideoData*)_this->driverdata;
+
+    if (SDL_GetHintBoolean(SDL_HINT_WINDOWS_DPI_SCALING, SDL_FALSE)) {
+        WIN_DeclareDPIAwarePerMonitorV2(_this);
+
+        data->dpi_scaling_enabled = SDL_TRUE;
+    }
+}
+
 int
 WIN_VideoInit(_THIS)
 {
     SDL_VideoData *data = (SDL_VideoData *) _this->driverdata;
 
     WIN_InitDPIAwareness(_this);
+    WIN_InitDPIScaling(_this);
 
 #ifdef HIGHDPI_DEBUG
     SDL_Log("DPI awareness: %s", WIN_GetDPIAwareness(_this));

src/video/windows/SDL_windowsvideo.h

@@ -188,6 +188,8 @@ typedef struct SDL_VideoData
                                         UINT             *dpiY );
     HRESULT (WINAPI *SetProcessDpiAwareness)(PROCESS_DPI_AWARENESS dpiAwareness);
 
+    SDL_bool dpi_scaling_enabled;
+
     SDL_bool ime_com_initialized;
     struct ITfThreadMgr *ime_threadmgr;
     SDL_bool ime_initialized;

src/video/windows/SDL_windowswindow.c

@@ -24,6 +24,7 @@
 
 #include "../../core/windows/SDL_windows.h"
 
+#include "SDL_log.h"
 #include "../SDL_sysvideo.h"
 #include "../SDL_pixels_c.h"
 #include "../../events/SDL_keyboard_c.h"
@@ -115,19 +116,45 @@ GetWindowStyle(SDL_Window * window)
     return style;
 }
 
+/**
+ * Returns arguments to pass to SetWindowPos - the window rect, including frame, in Windows coordinates.
+ * 
+ * Can be called before we have a HWND.
+ */
 static void
 WIN_AdjustWindowRectWithStyle(SDL_Window *window, DWORD style, BOOL menu, int *x, int *y, int *width, int *height, SDL_bool use_current)
 {
     SDL_WindowData *data = (SDL_WindowData *)window->driverdata;
     SDL_VideoData* videodata = SDL_GetVideoDevice() ? SDL_GetVideoDevice()->driverdata : NULL;
     RECT rect;
-    UINT dpi;
+    int dpi;
+    UINT frame_dpi;
 
-    dpi = 96;
+    /* Client rect, in SDL screen coordinates */
+    *x = (use_current ? window->x : window->windowed.x);
+    *y = (use_current ? window->y : window->windowed.y);
+    *width = (use_current ? window->w : window->windowed.w);
+    *height = (use_current ? window->h : window->windowed.h);
+
+    /* Convert client rect from SDL coordinates to pixels (no-op if DPI scaling not enabled) */
+    WIN_ScreenPointFromSDL(x, y, &dpi);
+    /* Note, use the guessed DPI returned from WIN_ScreenPointFromSDL rather than the cached one in
+       data->scaling_dpi.
+       
+       - This is called before the window is created, so we can't rely on data->scaling_dpi
+       - Bug workaround: when leaving exclusive fullscreen, the cached DPI and window DPI reported
+         by GetDpiForWindow will be wrong, and would cause windows shrinking slightly when
+         going from exclusive fullscreen to windowed on a HighDPI monitor with scaling if we used them.
+    */  
+    *width = MulDiv(*width, dpi, 96);
+    *height = MulDiv(*height, dpi, 96);
+
+    /* Copy the client size in pixels into this rect structure,
+       which we'll then adjust with AdjustWindowRectEx */
     rect.left = 0;
     rect.top = 0;
-    rect.right = (use_current ? window->w : window->windowed.w);
+    rect.right = *width;
-    rect.bottom = (use_current ? window->h : window->windowed.h);
+    rect.bottom = *height;
 
     /* borderless windows will have WM_NCCALCSIZE return 0 for the non-client area. When this happens, it looks like windows will send a resize message
        expanding the window client area to the previous window + chrome size, so shouldn't need to adjust the window size for the set styles.
@@ -141,26 +168,27 @@ WIN_AdjustWindowRectWithStyle(SDL_Window *window, DWORD style, BOOL menu, int *x
             RECT screen_rect;
             HMONITOR mon;
 
-            screen_rect.left = (use_current ? window->x : window->windowed.x);
+            screen_rect.left = *x;
-            screen_rect.top = (use_current ? window->y : window->windowed.y);
+            screen_rect.top = *y;
-            screen_rect.right = screen_rect.left + (use_current ? window->w : window->windowed.w);
+            screen_rect.right = *x + *width;
-            screen_rect.bottom  = screen_rect.top + (use_current ? window->h : window->windowed.h);
+            screen_rect.bottom  = *y + *height;
 
             mon = MonitorFromRect(&screen_rect, MONITOR_DEFAULTTONEAREST);
 
             /* GetDpiForMonitor docs promise to return the same hdpi / vdpi */
-            if (videodata->GetDpiForMonitor(mon, MDT_EFFECTIVE_DPI, &dpi, &unused) != S_OK) {
+            if (videodata->GetDpiForMonitor(mon, MDT_EFFECTIVE_DPI, &frame_dpi, &unused) != S_OK) {
-                dpi = 96;
+                frame_dpi = 96;
             }
 
-            videodata->AdjustWindowRectExForDpi(&rect, style, menu, 0, dpi);
+            videodata->AdjustWindowRectExForDpi(&rect, style, menu, 0, frame_dpi);
         } else {
             AdjustWindowRectEx(&rect, style, menu, 0);
         }
     }
 
-    *x = (use_current ? window->x : window->windowed.x) + rect.left;
+    /* Final rect in Windows screen space, including the frame */
-    *y = (use_current ? window->y : window->windowed.y) + rect.top;
+    *x += rect.left;
+    *y += rect.top;
     *width = (rect.right - rect.left);
     *height = (rect.bottom - rect.top);
 
@@ -170,7 +198,7 @@ WIN_AdjustWindowRectWithStyle(SDL_Window *window, DWORD style, BOOL menu, int *x
         (use_current ? window->y : window->windowed.y),
         (use_current ? window->w : window->windowed.w),
         (use_current ? window->h : window->windowed.h),
-        *x, *y, *width, *height, dpi);
+        *x, *y, *width, *height, frame_dpi);
 #endif
 }
 
@@ -217,6 +245,43 @@ WIN_MouseRelativeModeCenterChanged(void *userdata, const char *name, const char
     data->mouse_relative_mode_center = SDL_GetStringBoolean(hint, SDL_TRUE);
 }
 
+static int
+WIN_GetScalingDPIForHWND(const SDL_VideoData *videodata, HWND hwnd)
+{
+    /* DPI scaling not requested? */
+    if (!videodata->dpi_scaling_enabled) {
+        return 96;
+    }
+
+    /* Window 10+ */
+    if (videodata->GetDpiForWindow) {
+        return videodata->GetDpiForWindow(hwnd);
+    }
+
+    /* Window 8.1+ */
+    if (videodata->GetDpiForMonitor) {
+        HMONITOR monitor = MonitorFromWindow(hwnd, MONITOR_DEFAULTTONEAREST);
+        if (monitor) {
+            UINT dpi_uint, unused;
+            if (S_OK == videodata->GetDpiForMonitor(monitor, MDT_EFFECTIVE_DPI, &dpi_uint, &unused)) {
+                return (int)dpi_uint;
+            }
+        }
+        return 96;
+    }
+
+    /* Windows Vista-8.0 */
+    {
+        HDC hdc = GetDC(NULL);
+        if (hdc) {
+            int dpi = GetDeviceCaps(hdc, LOGPIXELSX);
+            ReleaseDC(NULL, hdc);
+            return dpi;
+        }
+        return 96;
+    }
+}
+
 static int
 SetupWindowData(_THIS, SDL_Window * window, HWND hwnd, HWND parent, SDL_bool created)
 {
@@ -239,6 +304,11 @@ SetupWindowData(_THIS, SDL_Window * window, HWND hwnd, HWND parent, SDL_bool cre
     data->last_pointer_update = (LPARAM)-1;
     data->videodata = videodata;
     data->initializing = SDL_TRUE;
+    data->scaling_dpi = WIN_GetScalingDPIForHWND(videodata, hwnd);
+
+#ifdef HIGHDPI_DEBUG
+    SDL_Log("SetupWindowData: initialized data->scaling_dpi to %d", data->scaling_dpi);
+#endif
 
     SDL_AddHintCallback(SDL_HINT_MOUSE_RELATIVE_MODE_CENTER, WIN_MouseRelativeModeCenterChanged, data);
 
@@ -274,6 +344,8 @@ SetupWindowData(_THIS, SDL_Window * window, HWND hwnd, HWND parent, SDL_bool cre
         if (GetClientRect(hwnd, &rect)) {
             int w = rect.right;
             int h = rect.bottom;
+
+            WIN_ClientPointToSDL(window, &w, &h);
             if ((window->windowed.w && window->windowed.w != w) || (window->windowed.h && window->windowed.h != h)) {
                 /* We tried to create a window larger than the desktop and Windows didn't allow it.  Override! */
                 int x, y;
@@ -293,8 +365,11 @@ SetupWindowData(_THIS, SDL_Window * window, HWND hwnd, HWND parent, SDL_bool cre
         point.x = 0;
         point.y = 0;
         if (ClientToScreen(hwnd, &point)) {
-            window->x = point.x;
+            int x = point.x;
-            window->y = point.y;
+            int y = point.y;
+            WIN_ScreenPointToSDL(&x, &y);
+            window->x = x;
+            window->y = y;
         }
     }
     {
@@ -342,6 +417,11 @@ SetupWindowData(_THIS, SDL_Window * window, HWND hwnd, HWND parent, SDL_bool cre
         videodata->RegisterTouchWindow(hwnd, (TWF_FINETOUCH|TWF_WANTPALM));
     }
 
+    /* Force the SDL_WINDOW_ALLOW_HIGHDPI window flag if we are doing DPI scaling */
+    if (videodata->dpi_scaling_enabled) {
+        window->flags |= SDL_WINDOW_ALLOW_HIGHDPI;
+    }
+
     data->initializing = SDL_FALSE;
 
     /* All done! */
@@ -767,12 +847,17 @@ WIN_RestoreWindow(_THIS, SDL_Window * window)
     data->expected_resize = SDL_FALSE;
 }
 
+/**
+ * Reconfigures the window to fill the given display, if fullscreen is true, otherwise restores the window.
+ */
 void
 WIN_SetWindowFullscreen(_THIS, SDL_Window * window, SDL_VideoDisplay * display, SDL_bool fullscreen)
 {
+    SDL_DisplayData *displaydata = (SDL_DisplayData *) display->driverdata;
     SDL_WindowData *data = (SDL_WindowData *) window->driverdata;
+    SDL_VideoData *videodata = data->videodata;
     HWND hwnd = data->hwnd;
-    SDL_Rect bounds;
+    MONITORINFO minfo;
     DWORD style;
     HWND top;
     int x, y;
@@ -785,6 +870,15 @@ WIN_SetWindowFullscreen(_THIS, SDL_Window * window, SDL_VideoDisplay * display,
         return;
     }
 
+#ifdef HIGHDPI_DEBUG
+    SDL_Log("WIN_SetWindowFullscreen: %d", (int)fullscreen);
+#endif
+
+    /* Clear the window size, to force SDL_SendWindowEvent to send a SDL_WINDOWEVENT_RESIZED
+       event in WM_WINDOWPOSCHANGED. */
+    data->window->w = 0;
+    data->window->h = 0;
+
     if (SDL_ShouldAllowTopmost() && (window->flags & SDL_WINDOW_ALWAYS_ON_TOP)) {
         top = HWND_TOPMOST;
     } else {
@@ -795,13 +889,20 @@ WIN_SetWindowFullscreen(_THIS, SDL_Window * window, SDL_VideoDisplay * display,
     style &= ~STYLE_MASK;
     style |= GetWindowStyle(window);
 
-    WIN_GetDisplayBounds(_this, display, &bounds);
+    /* Use GetMonitorInfo instead of WIN_GetDisplayBounds because we want the
+       monitor bounds in Windows coordinates (pixels) rather than SDL coordinates (points). */
+    SDL_zero(minfo);
+    minfo.cbSize = sizeof(MONITORINFO);
+    if (!GetMonitorInfo(displaydata->MonitorHandle, &minfo)) {
+        SDL_SetError("GetMonitorInfo failed");
+        return;
+    }
 
     if (fullscreen) {
-        x = bounds.x;
+        x = minfo.rcMonitor.left;
-        y = bounds.y;
+        y = minfo.rcMonitor.top;
-        w = bounds.w;
+        w = minfo.rcMonitor.right - minfo.rcMonitor.left;
-        h = bounds.h;
+        h = minfo.rcMonitor.bottom - minfo.rcMonitor.top;
 
         /* Unset the maximized flag.  This fixes
            https://bugzilla.libsdl.org/show_bug.cgi?id=3215
@@ -831,6 +932,10 @@ WIN_SetWindowFullscreen(_THIS, SDL_Window * window, SDL_VideoDisplay * display,
     data->expected_resize = SDL_TRUE;
     SetWindowPos(hwnd, top, x, y, w, h, SWP_NOCOPYBITS | SWP_NOACTIVATE);
     data->expected_resize = SDL_FALSE;
+
+#ifdef HIGHDPI_DEBUG
+    SDL_Log("WIN_SetWindowFullscreen: %d finished. Set window to %d,%d, %dx%d", (int)fullscreen, x, y, w, h);
+#endif
 }
 
 int
@@ -1122,12 +1227,19 @@ WIN_UpdateClipCursor(SDL_Window *window)
                 ClientToScreen(data->hwnd, (LPPOINT) & rect);
                 ClientToScreen(data->hwnd, (LPPOINT) & rect + 1);
                 if (window->mouse_rect.w > 0 && window->mouse_rect.h > 0) {
+                    SDL_Rect mouse_rect_win_client;
                     RECT mouse_rect, intersection;
 
-                    mouse_rect.left = rect.left + window->mouse_rect.x;
+                    /* mouse_rect_win_client is the mouse rect in Windows client space */
-                    mouse_rect.top = rect.top + window->mouse_rect.y;
+                    mouse_rect_win_client = window->mouse_rect;
-                    mouse_rect.right = mouse_rect.left + window->mouse_rect.w - 1;
+                    WIN_ClientPointFromSDL(window, &mouse_rect_win_client.x, &mouse_rect_win_client.y);
-                    mouse_rect.bottom = mouse_rect.top + window->mouse_rect.h - 1;
+                    WIN_ClientPointFromSDL(window, &mouse_rect_win_client.w, &mouse_rect_win_client.h);
+
+                    /* mouse_rect is the rect in Windows screen space */
+                    mouse_rect.left = rect.left + mouse_rect_win_client.x;
+                    mouse_rect.top = rect.top + mouse_rect_win_client.y;
+                    mouse_rect.right = mouse_rect.left + mouse_rect_win_client.w - 1;
+                    mouse_rect.bottom = mouse_rect.top + mouse_rect_win_client.h - 1;
                     if (IntersectRect(&intersection, &rect, &mouse_rect)) {
                         SDL_memcpy(&rect, &intersection, sizeof(rect));
                     } else if ((window->flags & SDL_WINDOW_MOUSE_GRABBED) != 0) {
@@ -1203,6 +1315,61 @@ WIN_SetWindowOpacity(_THIS, SDL_Window * window, float opacity)
     return 0;
 }
 
+/**
+ * Returns the drawable size in pixels (GetClientRect).
+ */
+void
+WIN_GetDrawableSize(const SDL_Window *window, int *w, int *h)
+{
+    const SDL_WindowData *data = ((SDL_WindowData *)window->driverdata);
+    HWND hwnd = data->hwnd;
+    RECT rect;
+
+    if (GetClientRect(hwnd, &rect)) {
+        *w = rect.right;
+        *h = rect.bottom;
+    } else {
+        *w = 0;
+        *h = 0;
+    }
+}
+
+/**
+ * Convert a point in the client area from pixels to DPI-scaled points.
+ * 
+ * No-op if DPI scaling is not enabled.
+ */
+void
+WIN_ClientPointToSDL(const SDL_Window *window, int *x, int *y)
+{
+    const SDL_WindowData *data = ((SDL_WindowData *)window->driverdata);
+    const SDL_VideoData *videodata = data->videodata;
+
+    if (!videodata->dpi_scaling_enabled)
+        return;
+
+    *x = MulDiv(*x, 96, data->scaling_dpi);
+    *y = MulDiv(*y, 96, data->scaling_dpi);
+}
+
+/**
+ * Convert a point in the client area from DPI-scaled points to pixels.
+ * 
+ * No-op if DPI scaling is not enabled.
+ */
+void
+WIN_ClientPointFromSDL(const SDL_Window *window, int *x, int *y)
+{
+    const SDL_WindowData *data = ((SDL_WindowData *)window->driverdata);
+    const SDL_VideoData *videodata = data->videodata;
+
+    if (!videodata->dpi_scaling_enabled)
+        return;
+    
+    *x = MulDiv(*x, data->scaling_dpi, 96);
+    *y = MulDiv(*y, data->scaling_dpi, 96);
+}
+
 void
 WIN_AcceptDragAndDrop(SDL_Window * window, SDL_bool accept)
 {

src/video/windows/SDL_windowswindow.h

@@ -59,6 +59,11 @@ typedef struct
 #if SDL_VIDEO_OPENGL_EGL  
     EGLSurface egl_surface;
 #endif
+    /**
+     * Cached value of GetDpiForWindow, for use for scaling points in the client area
+     * between dpi-scaled points and pixels. Only used if videodata->dpi_scaling_enabled.
+     */
+    int scaling_dpi;
 } SDL_WindowData;
 
 extern int WIN_CreateWindow(_THIS, SDL_Window * window);
@@ -91,6 +96,9 @@ extern SDL_bool WIN_GetWindowWMInfo(_THIS, SDL_Window * window,
 extern void WIN_OnWindowEnter(_THIS, SDL_Window * window);
 extern void WIN_UpdateClipCursor(SDL_Window *window);
 extern int WIN_SetWindowHitTest(SDL_Window *window, SDL_bool enabled);
+extern void WIN_GetDrawableSize(const SDL_Window *window, int *w, int *h);
+extern void WIN_ClientPointToSDL(const SDL_Window *window, int *w, int *h);
+extern void WIN_ClientPointFromSDL(const SDL_Window *window, int *w, int *h);
 extern void WIN_AcceptDragAndDrop(SDL_Window * window, SDL_bool accept);
 extern int WIN_FlashWindow(_THIS, SDL_Window * window, SDL_FlashOperation operation);