SDL: Only recreate SDL2 window when necessary

Destroying and recreating the SDL window whenever the video mode
changes in SDL2 is not necessary and causes several problems:

1. In windowed mode, the game window shifts position;
2. In fullscreen mode in macOS, every time the window is
   recreated, it causes the OS to play its switch-to-fullscreen
   animation again and emit system alert noises;
3. The window content flickers; and
4. The engine loses events from the old destroyed window.

This patch changes the SDL backend code to avoid destroying and
recreating the SDL window when using SDL2, except when switching
OpenGL modes, since there is no way to change the OpenGL feature
of a window.

There are still some outstanding issues with OpenGL where window
size ends up getting reset even though the user has resized it;
this will probably need to be addressed at some point in another
patch.

Thanks to @bgK and @criezy for their feedback which made this
patch much better.

Co-Authored-By: Bastien Bouclet <bastien.bouclet@gmail.com>

1 files changed, 19 insertions, 11 deletions

diff --git a/backends/graphics/openglsdl/openglsdl-graphics.cpp b/backends/graphics/openglsdl/openglsdl-graphics.cpp
index e41148062f..f664314862 100644
--- a/backends/graphics/openglsdl/openglsdl-graphics.cpp
+++ b/backends/graphics/openglsdl/openglsdl-graphics.cpp
@@ -463,10 +463,6 @@ bool OpenGLSdlGraphicsManager::setupMode(uint width, uint height) {
 		// Remember our choice.
 		ConfMan.setInt("last_fullscreen_mode_width", _desiredFullscreenWidth, Common::ConfigManager::kApplicationDomain);
 		ConfMan.setInt("last_fullscreen_mode_height", _desiredFullscreenHeight, Common::ConfigManager::kApplicationDomain);
-
-		// Use our choice.
-		width  = _desiredFullscreenWidth;
-		height = _desiredFullscreenHeight;
 	}
 
 	// This is pretty confusing since RGBA8888 talks about the memory
@@ -489,13 +485,23 @@ bool OpenGLSdlGraphicsManager::setupMode(uint width, uint height) {
 		_glContext = nullptr;
 	}
 
-	_window->destroyWindow();
-
-	uint32 flags = SDL_WINDOW_OPENGL;
+	uint32 flags = SDL_WINDOW_OPENGL | SDL_WINDOW_RESIZABLE;
 	if (_wantsFullScreen) {
+		// On Linux/X11, when toggling to fullscreen, the window manager saves
+		// the window size to be able to restore it when going back to windowed mode.
+		// If the user configured ScummVM to start in fullscreen mode, we first
+		// create a window and then toggle it to fullscreen to give the window manager
+		// a chance to save the window size. That way if the user switches back
+		// to windowed mode, the window manager has a window size to apply instead
+		// of leaving the window at the fullscreen resolution size.
+		if (!_window->getSDLWindow()) {
+			_window->createOrUpdateWindow(width, height, flags);
+		}
+
+		width  = _desiredFullscreenWidth;
+		height = _desiredFullscreenHeight;
+
 		flags |= SDL_WINDOW_FULLSCREEN;
-	} else {
-		flags |= SDL_WINDOW_RESIZABLE;
 	}
 
 	// Request a OpenGL (ES) context we can use.
@@ -503,11 +509,11 @@ bool OpenGLSdlGraphicsManager::setupMode(uint width, uint height) {
 	SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, _glContextMajor);
 	SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, _glContextMinor);
 
-	if (!_window->createWindow(width, height, flags)) {
+	if (!_window->createOrUpdateWindow(width, height, flags)) {
 		// We treat fullscreen requests as a "hint" for now. This means in
 		// case it is not available we simply ignore it.
 		if (_wantsFullScreen) {
-			_window->createWindow(width, height, SDL_WINDOW_OPENGL | SDL_WINDOW_RESIZABLE);
+			_window->createOrUpdateWindow(width, height, SDL_WINDOW_OPENGL | SDL_WINDOW_RESIZABLE);
 		}
 
 		if (!_window->getSDLWindow()) {
@@ -541,6 +547,8 @@ bool OpenGLSdlGraphicsManager::setupMode(uint width, uint height) {
 
 	uint32 flags = SDL_OPENGL;
 	if (_wantsFullScreen) {
+		width  = _desiredFullscreenWidth;
+		height = _desiredFullscreenHeight;
 		flags |= SDL_FULLSCREEN;
 	} else {
 		flags |= SDL_RESIZABLE;