wayland: Get the window content scale from the backend

The window content scale may change independently of the display it is on if scaling or accessibility features are involved, so query it directly from the backend instead of inferring it.

src/video/SDL_sysvideo.h

@@ -264,6 +264,7 @@ struct SDL_VideoDevice
     void (*SetWindowMaximumSize)(SDL_VideoDevice *_this, SDL_Window *window);
     void (*SetWindowAspectRatio)(SDL_VideoDevice *_this, SDL_Window *window);
     bool (*GetWindowBordersSize)(SDL_VideoDevice *_this, SDL_Window *window, int *top, int *left, int *bottom, int *right);
+    float (*GetWindowContentScale)(SDL_VideoDevice *_this, SDL_Window *window);
     void (*GetWindowSizeInPixels)(SDL_VideoDevice *_this, SDL_Window *window, int *w, int *h);
     bool (*SetWindowOpacity)(SDL_VideoDevice *_this, SDL_Window *window, float opacity);
     bool (*SetWindowParent)(SDL_VideoDevice *_this, SDL_Window *window, SDL_Window *parent);

src/video/SDL_video.c

@@ -1751,11 +1751,17 @@ float SDL_GetWindowDisplayScale(SDL_Window *window)
 
 static void SDL_CheckWindowDisplayScaleChanged(SDL_Window *window)
 {
-    float pixel_density = SDL_GetWindowPixelDensity(window);
-    float content_scale = SDL_GetDisplayContentScale(SDL_GetDisplayForWindowPosition(window));
     float display_scale;
 
-    display_scale = (pixel_density * content_scale);
+    if (_this->GetWindowContentScale) {
+        display_scale = _this->GetWindowContentScale(_this, window);
+    } else {
+        const float pixel_density = SDL_GetWindowPixelDensity(window);
+        const float content_scale = SDL_GetDisplayContentScale(SDL_GetDisplayForWindowPosition(window));
+
+        display_scale = pixel_density * content_scale;
+    }
+
     if (display_scale != window->display_scale) {
         window->display_scale = display_scale;
         SDL_SendWindowEvent(window, SDL_EVENT_WINDOW_DISPLAY_SCALE_CHANGED, 0, 0);

src/video/wayland/SDL_waylandvideo.c

@@ -625,6 +625,7 @@ static SDL_VideoDevice *Wayland_CreateDevice(bool require_preferred_protocols)
     device->SetWindowTitle = Wayland_SetWindowTitle;
     device->SetWindowIcon = Wayland_SetWindowIcon;
     device->GetWindowSizeInPixels = Wayland_GetWindowSizeInPixels;
+    device->GetWindowContentScale = Wayland_GetWindowContentScale;
     device->GetDisplayForWindow = Wayland_GetDisplayForWindow;
     device->DestroyWindow = Wayland_DestroyWindow;
     device->SetWindowHitTest = Wayland_SetWindowHitTest;

src/video/wayland/SDL_waylandwindow.c

@@ -2658,6 +2658,17 @@ void Wayland_GetWindowSizeInPixels(SDL_VideoDevice *_this, SDL_Window *window, i
     *h = data->current.pixel_height;
 }
 
+float Wayland_GetWindowContentScale(SDL_VideoDevice *_this, SDL_Window *window)
+{
+    SDL_WindowData *wind = window->internal;
+
+    if (window->flags & SDL_WINDOW_HIGH_PIXEL_DENSITY || wind->scale_to_display || wind->fullscreen_exclusive) {
+        return (float)wind->scale_factor;
+    }
+
+    return 1.0f;
+}
+
 SDL_DisplayID Wayland_GetDisplayForWindow(SDL_VideoDevice *_this, SDL_Window *window)
 {
     SDL_WindowData *wind = window->internal;

src/video/wayland/SDL_waylandwindow.h

@@ -208,6 +208,7 @@ extern void Wayland_ShowWindowSystemMenu(SDL_Window *window, int x, int y);
 extern void Wayland_DestroyWindow(SDL_VideoDevice *_this, SDL_Window *window);
 extern bool Wayland_SuspendScreenSaver(SDL_VideoDevice *_this);
 extern bool Wayland_SetWindowIcon(SDL_VideoDevice *_this, SDL_Window *window, SDL_Surface *icon);
+extern float Wayland_GetWindowContentScale(SDL_VideoDevice *_this, SDL_Window *window);
 
 extern bool Wayland_SetWindowHitTest(SDL_Window *window, bool enabled);
 extern bool Wayland_FlashWindow(SDL_VideoDevice *_this, SDL_Window *window, SDL_FlashOperation operation);