In Qt5 and Qt6, when switching a QQuickView-based application from fullscreen mode to normal mode and then resizing it, unknown subwindows are rendered behind the QQuickView.

Update:

During scaling, the QQuickView glitches and doesn't resize properly to the application window.

Additionally, when dragging the cursor across the black area, the cursor is rendered multiple times. Unfortunately, the screen recorder was unable to capture this glitch.