SDL: video: Store the preferred fullscreen display in a property for sdl2-compat

SDL Commits
1 
From c5d5967c3a435748733536e1061de76864eed214 Mon Sep 17 00:00:00 2001
From: Frank Praznik <[EMAIL REDACTED]>
Date: Wed, 30 Apr 2025 11:21:09 -0400
Subject: [PATCH] video: Store the preferred fullscreen display in a property
 for sdl2-compat

sdl2-compat will call SDL_GetDisplayForWindow() when querying the display to use for fullscreen, which won't always be correct if the backend can't actually reposition the window. When calling this function, get the ideal fullscreen display and store it in a property for retrieval by sdl2-compat.
---
 src/video/SDL_video.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/src/video/SDL_video.c b/src/video/SDL_video.c
index 26ae67bc2a87c..c9eb1caa75c40 100644
--- a/src/video/SDL_video.c
+++ b/src/video/SDL_video.c
@@ -1731,12 +1731,25 @@ SDL_VideoDisplay *SDL_GetVideoDisplayForFullscreenWindow(SDL_Window *window)
     return SDL_GetVideoDisplay(displayID);
 }
 
+#define SDL_PROP_SDL2_COMPAT_WINDOW_PREFERRED_FULLSCREEN_DISPLAY "sdl2-compat.window.preferred_fullscreen_display"
+
 SDL_DisplayID SDL_GetDisplayForWindow(SDL_Window *window)
 {
     SDL_DisplayID displayID = 0;
 
     CHECK_WINDOW_MAGIC(window, 0);
 
+    /* sdl2-compat calls this function to get a display on which to make the window fullscreen,
+     * so pass it the preferred fullscreen display ID in a property.
+     */
+    SDL_PropertiesID window_props = SDL_GetWindowProperties(window);
+    SDL_VideoDisplay *fs_display = SDL_GetVideoDisplayForFullscreenWindow(window);
+    if (fs_display) {
+        SDL_SetNumberProperty(window_props, SDL_PROP_SDL2_COMPAT_WINDOW_PREFERRED_FULLSCREEN_DISPLAY, fs_display->id);
+    } else {
+        SDL_ClearProperty(window_props, SDL_PROP_SDL2_COMPAT_WINDOW_PREFERRED_FULLSCREEN_DISPLAY);
+    }
+
     // An explicit fullscreen display overrides all
     if (window->flags & SDL_WINDOW_FULLSCREEN) {
         displayID = window->current_fullscreen_mode.displayID;