wayland: Fall-back to the compositor for fullscreen-desktop window placement

Only use the specified output if an exclusive mode is being used, or a position was explicitly requested before entering fullscreen desktop. Otherwise, let the compositor handle placement, as it has more information about where the window is and where it should go, especially if fullscreen was requested before the window was fully mapped, or the window spans multiple outputs.

src/video/wayland/SDL_waylandwindow.c

@@ -589,7 +589,7 @@ static void Wayland_move_window(SDL_Window *window)
     }
 }
 
-static void SetFullscreen(SDL_Window *window, struct wl_output *output)
+static void SetFullscreen(SDL_Window *window, struct wl_output *output, bool fullscreen)
 {
     SDL_WindowData *wind = window->internal;
     SDL_VideoData *viddata = wind->waylandData;
@@ -602,7 +602,7 @@ static void SetFullscreen(SDL_Window *window, struct wl_output *output)
 
         wind->fullscreen_exclusive = output ? window->fullscreen_exclusive : false;
         ++wind->fullscreen_deadline_count;
-        if (output) {
+        if (fullscreen) {
             Wayland_SetWindowResizable(SDL_GetVideoDevice(), window, true);
             wl_surface_commit(wind->surface);
 
@@ -619,7 +619,7 @@ static void SetFullscreen(SDL_Window *window, struct wl_output *output)
 
         wind->fullscreen_exclusive = output ? window->fullscreen_exclusive : false;
         ++wind->fullscreen_deadline_count;
-        if (output) {
+        if (fullscreen) {
             Wayland_SetWindowResizable(SDL_GetVideoDevice(), window, true);
             wl_surface_commit(wind->surface);
 
@@ -655,7 +655,7 @@ static void UpdateWindowFullscreen(SDL_Window *window, bool fullscreen)
                 SDL_VideoDisplay *disp = SDL_GetVideoDisplay(window->current_fullscreen_mode.displayID);
                 if (disp) {
                     wind->fullscreen_was_positioned = true;
-                    SetFullscreen(window, disp->internal->output);
+                    SetFullscreen(window, disp->internal->output, true);
                 }
             }
         }
@@ -2314,7 +2314,19 @@ SDL_FullscreenResult Wayland_SetWindowFullscreen(SDL_VideoDevice *_this, SDL_Win
     // Don't send redundant fullscreen set/unset events.
     if (!!fullscreen != wind->is_fullscreen) {
         wind->fullscreen_was_positioned = !!fullscreen;
-        SetFullscreen(window, fullscreen ? output : NULL);
+
+        /* Only use the specified output if an exclusive mode is being used, or a position was explicitly requested
+         * before entering fullscreen desktop. Otherwise, let the compositor handle placement, as it has more
+         * information about where the window is and where it should go, particularly if fullscreen is being requested
+         * before the window is mapped, or the window spans multiple outputs.
+         */
+        if (!window->fullscreen_exclusive) {
+            if (window->undefined_x || window->undefined_y ||
+                (wind->num_outputs && !window->last_position_pending)) {
+                output = NULL;
+            }
+        }
+        SetFullscreen(window, output, !!fullscreen);
     } else if (wind->is_fullscreen) {
         /*
          * If the window is already fullscreen, this is likely a request to switch between
@@ -2325,7 +2337,7 @@ SDL_FullscreenResult Wayland_SetWindowFullscreen(SDL_VideoDevice *_this, SDL_Win
          */
         if (wind->last_displayID != display->id) {
             wind->fullscreen_was_positioned = true;
-            SetFullscreen(window, output);
+            SetFullscreen(window, output, true);
         } else {
             ConfigureWindowGeometry(window);
             CommitLibdecorFrame(window);
@@ -2759,7 +2771,7 @@ bool Wayland_SetWindowPosition(SDL_VideoDevice *_this, SDL_Window *window)
             SDL_VideoDisplay *display = SDL_GetVideoDisplayForFullscreenWindow(window);
             if (display && wind->last_displayID != display->id) {
                 struct wl_output *output = display->internal->output;
-                SetFullscreen(window, output);
+                SetFullscreen(window, output, true);
 
                 return true;
             }