video: Check the display origin when a fullscreen window is moved

In certain cases when moving fullscreen windows in scaled desktop configurations, the window origin might overlap two displays at once. Check if the window is at the origin of a specific display before falling back to the generic window rectangle check.

Fixes rare fullscreen window misplacement when moving fullscreen windows via a desktop shortcut while using the Wayland scale-to-display mode.

Author: Kontrabant

src/video/SDL_video.c

@@ -1674,6 +1674,21 @@ SDL_DisplayID SDL_GetDisplayForRect(const SDL_Rect *rect)
     return GetDisplayForRect(rect->x, rect->y, rect->w, rect->h);
 }
 
+static SDL_DisplayID GetDisplayAtOrigin(int x, int y)
+{
+    for (int i = 0; i < _this->num_displays; ++i) {
+        SDL_Rect rect;
+        const SDL_DisplayID cur_id = _this->displays[i]->id;
+        if (SDL_GetDisplayBounds(cur_id, &rect)) {
+            if (x == rect.x && y == rect.y) {
+                return cur_id;
+            }
+        }
+    }
+
+    return 0;
+}
+
 SDL_DisplayID SDL_GetDisplayForWindowPosition(SDL_Window *window)
 {
     int x, y;
@@ -1736,7 +1751,11 @@ SDL_VideoDisplay *SDL_GetVideoDisplayForFullscreenWindow(SDL_Window *window)
         const int w = window->last_size_pending ? window->pending.w : window->w;
         const int h = window->last_size_pending ? window->pending.h : window->h;
 
-        displayID = GetDisplayForRect(x, y, w, h);
+        // Check if the window is exactly at the origin of a display. Otherwise, fall back to the generic check.
+        displayID = GetDisplayAtOrigin(x, y);
+        if (!displayID) {
+            displayID = GetDisplayForRect(x, y, w, h);
+        }
     }
     if (!displayID) {
         // Use the primary display for a window if we can't find it anywhere else
@@ -2929,16 +2948,7 @@ bool SDL_SetWindowPosition(SDL_Window *window, int x, int y)
          * the pending fullscreen display ID if it does. This needs to be set early in case
          * the window is prevented from moving to the exact origin due to struts.
          */
-        for (int i = 0; i < _this->num_displays; ++i) {
-            SDL_Rect rect;
-            const SDL_DisplayID cur_id = _this->displays[i]->id;
-            if (SDL_GetDisplayBounds(cur_id, &rect)) {
-                if (x == rect.x && y == rect.y) {
-                    window->pending_displayID = cur_id;
-                    break;
-                }
-            }
-        }
+        window->pending_displayID = GetDisplayAtOrigin(x, y);
     }
 
     window->pending.x = x;