Bug 1761408 - Ensure Windows cursor is up-to-date from nsWindow::SetC…

…ursor(). r=mhowell

Also add better docs as to why we're doing what we're doing.

Also, avoid calling LoadCursor for built-in cursors over and over in chrome
pages.

Differential Revision: https://phabricator.services.mozilla.com/D142067

widget/windows/nsWindow.cpp

@@ -3245,26 +3245,34 @@ static HCURSOR CursorForImage(const nsIWidget::Cursor& aCursor,
   return cursor;
 }
 
-// Setting the actual cursor
 void nsWindow::SetCursor(const Cursor& aCursor) {
-  static HCURSOR sCustomHCursor = nullptr;
+  static HCURSOR sCurrentHCursor = nullptr;
+  static bool sCurrentHCursorIsCustom = false;
 
   mCursor = aCursor;
 
-  if (!mUpdateCursor && sCurrentCursor == aCursor) {
+  if (sCurrentCursor == aCursor && sCurrentHCursor && !mUpdateCursor) {
+    // Cursors in windows are global, so even if our mUpdateCursor flag is
+    // false we always need to make sure the Windows cursor is up-to-date,
+    // since stuff like native drag and drop / resizers code can mutate it
+    // outside of this method.
+    ::SetCursor(sCurrentHCursor);
     return;
   }
 
   mUpdateCursor = false;
-  if (sCustomHCursor) {
-    ::DestroyIcon(sCustomHCursor);
-    sCustomHCursor = nullptr;
-  }
 
+  if (sCurrentHCursorIsCustom) {
+    ::DestroyIcon(sCurrentHCursor);
+  }
+  sCurrentHCursor = nullptr;
+  sCurrentHCursorIsCustom = false;
   sCurrentCursor = aCursor;
+
   HCURSOR cursor = CursorForImage(aCursor, GetDefaultScale());
+  bool custom = false;
   if (cursor) {
-    sCustomHCursor = cursor;
+    custom = true;
   } else {
     cursor = CursorFor(aCursor.mDefaultCursor);
   }
@@ -3273,6 +3281,8 @@ void nsWindow::SetCursor(const Cursor& aCursor) {
     return;
   }
 
+  sCurrentHCursor = cursor;
+  sCurrentHCursorIsCustom = custom;
   ::SetCursor(cursor);
 }