In Qt Quick Controls 1 flat demo, menu reopens when clicking unrelated parts of the app

I loaded the "Qt Quick Extras - Flat" demo to try it out, then built the project.

• Using the Qt 5.15.0 MinGW x64 kit, the UI never shows up. I posted this to https://bugreports.qt.io/browse/QTBUG-81622 .
• Using the Qt 5.14.2 MinGW x64 kit, the UI shows up.

When I click the "Menu" button 4 times, the menu shows up the first 3 clicks but not the fourth.

If I click the "Menu" button once, then click elsewhere twice, the menu shows up when I click an unrelated part of the app.

If I click the "Menu" button once, move the mouse, and click again, the menu shows up. I can keep moving or click-dragging the mouse, clicking an unrelated part of the app, and the menu keeps showing up.

Video recording at qt quick menu bug.mp4.