cocoa: Warp mouse to center of window before enabling relative mouse.

This prevents the case where the mouse might be at the edge of the
window when enabling relative mode, which confuses macOS, at it
might believe the user is attempting to resize the window.

Fixes libsdl-org#6994.

(cherry picked from commit 2afb49b)

src/video/cocoa/SDL_cocoamouse.m

@@ -296,10 +296,16 @@ static int Cocoa_WarpMouse(SDL_Window *window, float x, float y)
 
 static int Cocoa_SetRelativeMouseMode(SDL_bool enabled)
 {
+    SDL_Window *window = SDL_GetKeyboardFocus();
     CGError result;
-    SDL_Window *window;
     SDL_CocoaWindowData *data;
     if (enabled) {
+        if (window) {
+            /* make sure the mouse isn't at the corner of the window, as this can confuse things if macOS thinks a window resize is happening on the first click. */
+            const CGPoint point = CGPointMake((float)(window->x + (window->w / 2)), (float)(window->y + (window->h / 2)));
+            Cocoa_HandleMouseWarp(point.x, point.y);
+            CGWarpMouseCursorPosition(point);
+        }
         DLog("Turning on.");
         result = CGAssociateMouseAndMouseCursorPosition(NO);
     } else {
@@ -313,7 +319,6 @@ static int Cocoa_SetRelativeMouseMode(SDL_bool enabled)
     /* We will re-apply the non-relative mode when the window gets focus, if it
      * doesn't have focus right now.
      */
-    window = SDL_GetKeyboardFocus();
     if (!window) {
         return 0;
     }