test if renderer re-initialization is really necessary

app/src/scrcpy.c

@@ -146,8 +146,10 @@ handle_event(SDL_Event *event, bool control) {
             switch (event->window.event) {
                 case SDL_WINDOWEVENT_SIZE_CHANGED:
 #ifdef HIDPI_SUPPORT
-                    LOGD("Reinitializing renderer");
+                    if (!screen_test_correct_hidpi_ratio(&screen)) {
-                    screen_init_renderer_and_texture(&screen);
+                      LOGW("Reinitializing renderer due to incorrect hidpi ratio");
+                      screen_init_renderer_and_texture(&screen);
+                    }
 #endif
                     // fall-through no break
                 case SDL_WINDOWEVENT_EXPOSED:

app/src/screen.c

@@ -47,7 +47,10 @@ set_window_size(struct screen *screen, struct size new_size) {
     } else {
         SDL_SetWindowSize(screen->window, new_size.width, new_size.height);
 #ifdef HIDPI_SUPPORT
-        screen_init_renderer_and_texture(screen);
+        if (!screen_test_correct_hidpi_ratio(screen)) {
+            LOGW("Reinitializing renderer due to incorrect hidpi ratio");
+            screen_init_renderer_and_texture(screen);
+        }
 #endif
     }
 }
@@ -164,6 +167,14 @@ screen_init_renderer_and_texture(struct screen *screen) {
         return false;
     }
 
+#ifdef HIDPI_SUPPORT
+    int window_w, window_h, renderer_w, renderer_h;
+    SDL_GetWindowSize(screen->window, &window_w, &window_h);
+    SDL_GetRendererOutputSize(screen->renderer, &renderer_w, &renderer_h);
+    screen->expected_hidpi_w_factor = renderer_w * 1000 / window_w;
+    screen->expected_hidpi_h_factor = renderer_h * 1000 / window_h;
+#endif
+
     screen->texture = create_texture(screen->renderer, screen->frame_size);
     if (!screen->texture) {
         LOGC("Could not create texture: %s", SDL_GetError());
@@ -214,6 +225,19 @@ screen_init_rendering(struct screen *screen, const char *window_title,
     return screen_init_renderer_and_texture(screen);
 }
 
+#ifdef HIDPI_SUPPORT
+bool
+screen_test_correct_hidpi_ratio(struct screen *screen) {
+  int window_w, window_h, renderer_w, renderer_h;
+  SDL_GetWindowSize(screen->window, &window_w, &window_h);
+  SDL_GetRendererOutputSize(screen->renderer, &renderer_w, &renderer_h);
+  int current_hidpi_w_factor = renderer_w * 1000 / window_w;
+  int current_hidpi_h_factor = renderer_h * 1000 / window_h;
+  return current_hidpi_w_factor == screen->expected_hidpi_w_factor &&
+         current_hidpi_h_factor == screen->expected_hidpi_h_factor;
+}
+#endif
+
 void
 screen_show_window(struct screen *screen) {
     SDL_ShowWindow(screen->window);

app/src/screen.h

@@ -20,6 +20,15 @@ struct screen {
     bool has_frame;
     bool fullscreen;
     bool no_window;
+#ifdef HIDPI_SUPPORT
+    // these values store the ratio between renderer pixel size and window size
+    // in most configurations these ratios would be 1.0 (1000), but on MacOS with
+    // a Retina/HIDPI monitor connected they are 2.0 (2000) because that's how
+    // Apple chose to maintain compatibility with legacy apps, by pretending that
+    // that the screen has half of its real resolution.
+    int expected_hidpi_w_factor; // multiplied by 1000 to avoid float
+    int expected_hidpi_h_factor; // multiplied by 1000 to avoid float
+#endif
 };
 
 #define SCREEN_INITIALIZER {  \
@@ -48,6 +57,15 @@ bool
 screen_init_rendering(struct screen *screen, const char *window_title,
                       struct size frame_size, bool always_on_top);
 
+#ifdef HIDPI_SUPPORT
+// test if the expected renderer to window ratio is correct
+// used to work around SDL bugs
+// returns true if correct.
+// If it returns false the renderer state needs to be fixed
+bool
+screen_test_correct_hidpi_ratio(struct screen *screen);
+#endif
+
 // reinitialize the renderer (only used in some configurations
 // if necessary to workaround SDL bugs)
 bool