Fixed bug 3015 - grab mouse inconsistent state

Martin Gerhardy

Not sure - but I think there might be a logic flaw in SDL_SetWindowGrab.

The problem here is that this modifies the window flags and e.g. sets
SDL_WINDOW_INPUT_GRABBED - but the _this->grabbed_window pointer is not
yet set.

Then in SDL_UpdateWindowGrab the _this->grabbed_window pointer is only
set if the function pointer _this->SetWindowGrab is not NULL. But if
this is NULL, the _this->grabbed_window pointer is never set, but every
future call to any of the grab functions include an assert for:
SDL_assert(!_this->grabbed_window || ((_this->grabbed_window->flags &
SDL_WINDOW_INPUT_GRABBED) != 0));

That means the first call works, the second always fails and triggers
the assert.

src/video/SDL_video.c

@@ -2122,28 +2122,30 @@ SDL_GetWindowGammaRamp(SDL_Window * window, Uint16 * red,
 void
 SDL_UpdateWindowGrab(SDL_Window * window)
 {
-    if (_this->SetWindowGrab) {
-        SDL_Window *grabbed_window;
-        SDL_bool grabbed;
-        if ((SDL_GetMouse()->relative_mode || (window->flags & SDL_WINDOW_INPUT_GRABBED)) &&
-             (window->flags & SDL_WINDOW_INPUT_FOCUS)) {
-            grabbed = SDL_TRUE;
-        } else {
-            grabbed = SDL_FALSE;
-        }
+    SDL_Window *grabbed_window;
+    SDL_bool grabbed;
+    if ((SDL_GetMouse()->relative_mode || (window->flags & SDL_WINDOW_INPUT_GRABBED)) &&
+         (window->flags & SDL_WINDOW_INPUT_FOCUS)) {
+        grabbed = SDL_TRUE;
+    } else {
+        grabbed = SDL_FALSE;
+    }
 
-        grabbed_window = _this->grabbed_window;
-        if (grabbed) {
-            if (grabbed_window && (grabbed_window != window)) {
-                /* stealing a grab from another window! */
-                grabbed_window->flags &= ~SDL_WINDOW_INPUT_GRABBED;
+    grabbed_window = _this->grabbed_window;
+    if (grabbed) {
+        if (grabbed_window && (grabbed_window != window)) {
+            /* stealing a grab from another window! */
+            grabbed_window->flags &= ~SDL_WINDOW_INPUT_GRABBED;
+            if (_this->SetWindowGrab) {
                 _this->SetWindowGrab(_this, grabbed_window, SDL_FALSE);
             }
-            _this->grabbed_window = window;
-        } else if (grabbed_window == window) {
-            _this->grabbed_window = NULL;  /* ungrabbing. */
         }
+        _this->grabbed_window = window;
+    } else if (grabbed_window == window) {
+        _this->grabbed_window = NULL;  /* ungrabbing. */
+    }
 
+    if (_this->SetWindowGrab) {
         _this->SetWindowGrab(_this, window, grabbed);
     }
 }