Fix mode switching due to information loss when casting the mode struct

When entering exclusive fullscreen, SDL3 verifies that the requested mode parameters precisely match a mode exposed by the output, which can cause the mode switch to fail due to the float to int rounding of the refresh rate when converting between the SDL2 and SDL3 mode structs. Instead of casting directly, use the SDL2 mode parameters to try and find the closest matching SDL3 display mode.

This fixes video mode switching for modes with refresh rates that are not precisely an integer, and allows for removing the DisplayMode_2to3 function entirely.

src/sdl2_compat.c

@@ -6107,19 +6107,6 @@ SDL_UnlockSensors(void)
     SDL3_UnlockMutex(sensor_lock);
 }
 
-
-static void
-DisplayMode_2to3(const SDL2_DisplayMode *in, SDL_DisplayMode *out) {
-    if (in && out) {
-        out->format = (SDL_PixelFormat)in->format;
-        out->w = in->w;
-        out->h = in->h;
-        out->refresh_rate = (float) in->refresh_rate;
-        out->pixel_density = 1.0f;
-        out->internal = (SDL_DisplayModeData *)in->driverdata;
-    }
-}
-
 static void
 DisplayMode_3to2(const SDL_DisplayMode *in, SDL2_DisplayMode *out) {
     if (in && out) {
@@ -6352,12 +6339,17 @@ SDL_GetDesktopDisplayMode(int displayIndex, SDL2_DisplayMode *mode)
 static int
 ApplyFullscreenMode(SDL_Window *window)
 {
-    SDL_DisplayMode *mode = (SDL_DisplayMode *) SDL3_GetPointerProperty(SDL3_GetWindowProperties(window), PROP_WINDOW_FULLSCREEN_MODE, NULL);
-    if (mode) {
+    SDL2_DisplayMode *property = (SDL2_DisplayMode *) SDL3_GetPointerProperty(SDL3_GetWindowProperties(window), PROP_WINDOW_FULLSCREEN_MODE, NULL);
+    SDL_DisplayMode mode;
+    SDL3_zero(mode);
+    if (property) {
         /* Always try to enter fullscreen on the current display */
-        mode->displayID = SDL3_GetDisplayForWindow(window);
+        const SDL_DisplayID displayID = SDL3_GetDisplayForWindow(window);
+
+        /* The SDL2 refresh rate is rounded off, and SDL3 checks that the mode parameters match exactly, so try to find the closest matching SDL3 mode. */
+        SDL3_GetClosestFullscreenDisplayMode(displayID, property->w, property->h, (float)property->refresh_rate, false, &mode);
     }
-    if (mode && SDL3_SetWindowFullscreenMode(window, mode)) {
+    if (mode.displayID && SDL3_SetWindowFullscreenMode(window, &mode)) {
         return 0;
     } else {
         int count = 0;
@@ -6387,7 +6379,7 @@ SDL_DECLSPEC int SDLCALL
 SDL_GetWindowDisplayMode(SDL_Window *window, SDL2_DisplayMode *mode)
 {
     /* returns a pointer to the fullscreen mode to use or NULL for desktop mode */
-    const SDL_DisplayMode *dp;
+    const SDL2_DisplayMode *dp;
 
     if (!window) {
         SDL3_SetError("Invalid window");
@@ -6399,22 +6391,23 @@ SDL_GetWindowDisplayMode(SDL_Window *window, SDL2_DisplayMode *mode)
         return -1;
     }
 
-    dp = (SDL_DisplayMode *) SDL3_GetPointerProperty(SDL3_GetWindowProperties(window), PROP_WINDOW_FULLSCREEN_MODE, (void *)SDL3_GetWindowFullscreenMode(window));
+    dp = (SDL2_DisplayMode *) SDL3_GetPointerProperty(SDL3_GetWindowProperties(window), PROP_WINDOW_FULLSCREEN_MODE, NULL);
     if (dp) {
-        DisplayMode_3to2(dp, mode);
+        SDL3_copyp(mode, dp);
     } else {
+        const SDL_DisplayMode *dp3;
         SDL_DisplayID displayID = SDL3_GetDisplayForWindow(window);
         if (!displayID) {
             displayID = SDL3_GetPrimaryDisplay();
         }
 
         /* Desktop mode */
         /* FIXME: is this correct ? */
-        dp = SDL3_GetDesktopDisplayMode(displayID);
-        if (dp == NULL) {
+        dp3 = SDL3_GetDesktopDisplayMode(displayID);
+        if (dp3 == NULL) {
             return -1;
         }
-        DisplayMode_3to2(dp, mode);
+        DisplayMode_3to2(dp3, mode);
 
         /* When returning the desktop mode, make sure the refresh is some nonzero value. */
         if (mode->refresh_rate == 0) {
@@ -6463,16 +6456,12 @@ SDL_DECLSPEC int SDLCALL
 SDL_SetWindowDisplayMode(SDL_Window *window, const SDL2_DisplayMode *mode)
 {
     int result;
-    SDL_DisplayMode dp;
-
-    SDL3_zero(dp);
-    DisplayMode_2to3(mode, &dp);
 
     /* Store the requested mode in case we aren't in full-screen exclusive mode yet (or ever) */
     if (mode) {
-        SDL_DisplayMode *property = (SDL_DisplayMode *) SDL3_malloc(sizeof(*property));
+        SDL2_DisplayMode *property = (SDL2_DisplayMode *) SDL3_malloc(sizeof(*property));
         if (property) {
-            SDL3_copyp(property, &dp);
+            SDL3_copyp(property, mode);
             result = SDL3_SetPointerPropertyWithCleanup(SDL3_GetWindowProperties(window), PROP_WINDOW_FULLSCREEN_MODE, property, CleanupFreeableProperty, NULL) ? 0 : -1;
         } else {
             result = -1;