wayland: Use the cached window size when switching from non-floating to floating window state

When changing the window state from non-floating to floating (e.g. leaving fullscreen), libdecor can send bogus content sizes that are +/- the height of the window title bar and start 'walking' the window height in one direction or the other with every transition.

The floating window size is known, so use the cached value instead of the size reported by libdecor when restoring the floating state.

src/video/wayland/SDL_waylandwindow.c

@@ -878,11 +878,17 @@ decoration_frame_configure(struct libdecor_frame *frame,
         wind->floating_resize_pending = SDL_FALSE;
     } else {
         /*
-         * XXX: When hiding a floating window, libdecor can send bogus content sizes that
-         *      are +/- the height of the title bar, which distorts the window size.
-         *      Ignore any values from libdecor when hiding a floating window.
+         * XXX: libdecor can send bogus content sizes that are +/- the height
+         *      of the title bar when hiding a window or transitioning from
+         *      non-floating to floating state, which distorts the window size.
+         *
+         *      Ignore any size values from libdecor in these scenarios in
+         *      favor of the cached window size.
+         *
+         *      https://gitlab.gnome.org/jadahl/libdecor/-/issues/40
          */
-        const SDL_bool use_cached_size = (window->is_hiding || !!(window->flags & SDL_WINDOW_HIDDEN));
+        const SDL_bool use_cached_size = (floating && !wind->was_floating) ||
+                                         (window->is_hiding || !!(window->flags & SDL_WINDOW_HIDDEN));
 
         /* This will never set 0 for width/height unless the function returns false */
         if (use_cached_size || !libdecor_configuration_get_content_size(configuration, frame, &width, &height)) {
@@ -905,6 +911,8 @@ decoration_frame_configure(struct libdecor_frame *frame,
         wind->floating_height = height;
     }
 
+    wind->was_floating = floating;
+
     /* Do the resize on the SDL side (this will set window->w/h)... */
     Wayland_HandleResize(window, width, height, scale_factor);
     wind->shell_surface.libdecor.initial_configure_seen = SDL_TRUE;

src/video/wayland/SDL_waylandwindow.h

@@ -102,6 +102,7 @@ typedef struct {
     int window_width, window_height;
     SDL_bool needs_resize_event;
     SDL_bool floating_resize_pending;
+    SDL_bool was_floating;
     SDL_bool is_fullscreen;
     SDL_bool in_fullscreen_transition;
     Uint32 fullscreen_flags;