Introduces Cocoa_GetWindowDisplayIndex. This enable a proper management for dpi when switching between retina and non-retina displays.

src/video/SDL_sysvideo.h

@@ -237,6 +237,7 @@ struct SDL_VideoDevice
     int (*SetWindowGammaRamp) (_THIS, SDL_Window * window, const Uint16 * ramp);
     int (*GetWindowGammaRamp) (_THIS, SDL_Window * window, Uint16 * ramp);
     void* (*GetWindowICCProfile) (_THIS, SDL_Window * window, size_t* size);
+    int (*GetWindowDisplayIndex)(_THIS, SDL_Window * window);
     void (*SetWindowMouseRect)(_THIS, SDL_Window * window);
     void (*SetWindowMouseGrab) (_THIS, SDL_Window * window, SDL_bool grabbed);
     void (*SetWindowKeyboardGrab) (_THIS, SDL_Window * window, SDL_bool grabbed);

src/video/SDL_video.c

@@ -1060,6 +1060,10 @@ SDL_GetDisplay(int displayIndex)
 int
 SDL_GetWindowDisplayIndex(SDL_Window * window)
 {
+    CHECK_WINDOW_MAGIC(window, -1);
+    if (_this->GetWindowDisplayIndex) {
+        return _this->GetWindowDisplayIndex(_this, window);
+    } else {
         int displayIndex;
         int i, dist;
         int closest = -1;
@@ -1068,8 +1072,6 @@ SDL_GetWindowDisplayIndex(SDL_Window * window)
         SDL_Point delta;
         SDL_Rect rect;
 
-    CHECK_WINDOW_MAGIC(window, -1);
-
         if (SDL_WINDOWPOS_ISUNDEFINED(window->x) ||
             SDL_WINDOWPOS_ISCENTERED(window->x)) {
             displayIndex = (window->x & 0xFFFF);
@@ -1116,6 +1118,7 @@ SDL_GetWindowDisplayIndex(SDL_Window * window)
         }
         return closest;
     }
+}
 
 SDL_VideoDisplay *
 SDL_GetDisplayForWindow(SDL_Window *window)

src/video/cocoa/SDL_cocoavideo.m

@@ -103,6 +103,7 @@ Cocoa_CreateDevice(int devindex)
     device->SetWindowGammaRamp = Cocoa_SetWindowGammaRamp;
     device->GetWindowGammaRamp = Cocoa_GetWindowGammaRamp;
     device->GetWindowICCProfile = Cocoa_GetWindowICCProfile;
+    device->GetWindowDisplayIndex = Cocoa_GetWindowDisplayIndex;
     device->SetWindowMouseRect = Cocoa_SetWindowMouseRect;
     device->SetWindowMouseGrab = Cocoa_SetWindowMouseGrab;
     device->SetWindowKeyboardGrab = Cocoa_SetWindowKeyboardGrab;

src/video/cocoa/SDL_cocoawindow.h

@@ -153,6 +153,7 @@ extern void Cocoa_SetWindowAlwaysOnTop(_THIS, SDL_Window * window, SDL_bool on_t
 extern void Cocoa_SetWindowFullscreen(_THIS, SDL_Window * window, SDL_VideoDisplay * display, SDL_bool fullscreen);
 extern int Cocoa_SetWindowGammaRamp(_THIS, SDL_Window * window, const Uint16 * ramp);
 extern void* Cocoa_GetWindowICCProfile(_THIS, SDL_Window * window, size_t * size);
+extern int Cocoa_GetWindowDisplayIndex(_THIS, SDL_Window * window);
 extern int Cocoa_GetWindowGammaRamp(_THIS, SDL_Window * window, Uint16 * ramp);
 extern void Cocoa_SetWindowMouseRect(_THIS, SDL_Window * window);
 extern void Cocoa_SetWindowMouseGrab(_THIS, SDL_Window * window, SDL_bool grabbed);

src/video/cocoa/SDL_cocoawindow.m

@@ -2197,6 +2197,36 @@ Cocoa_GetWindowICCProfile(_THIS, SDL_Window * window, size_t * size)
     return retIccProfileData;
 }
 
+int Cocoa_GetWindowDisplayIndex(_THIS, SDL_Window * window){
+    SDL_WindowData *data = (SDL_WindowData *) window->driverdata;
+
+    /* Not recognized via CHECK_WINDOW_MAGIC */
+    if (data == NULL){
+        return 0;
+    }
+
+    /*
+     Considering that we already have the display coordinates in which the window is placed (described via displayframe)
+     instead of checking in which display the window is placed, we should check which SDL display matches the display described
+     via displayframe.
+    */
+    CGRect displayframe = data->nswindow.screen.frame;
+    SDL_Point display_center;
+    SDL_Rect sdl_display_rect;
+    
+    display_center.x = displayframe.origin.x + displayframe.size.width / 2;
+    display_center.y = displayframe.origin.y + displayframe.size.height / 2;
+    
+    for (int i = 0; i < SDL_GetNumVideoDisplays(); i++){
+        SDL_GetDisplayBounds(i, &sdl_display_rect);
+        if (SDL_EnclosePoints(&display_center, 1, &sdl_display_rect, NULL)) {
+            return i;
+        }
+    }
+    SDL_SetError("Couldn't find the display where the window is attached to.");
+    return -1;
+}
+
 int
 Cocoa_GetWindowGammaRamp(_THIS, SDL_Window * window, Uint16 * ramp)
 {