Step 1. Click the button below and grant the window-management
permission. A popup will open and go fullscreen on a non-primary monitor.
Step 2. While the popup is fullscreen, do ONE of:
Press Win + Shift + ←/→ to move the fullscreen popup
to another monitor, then click button 2 below; OR
Physically unplug / disable the monitor the popup is on (the page
auto-detects this via screenschange).
Step 3. The page will close the popup. Focus then transfers back to
this Chrome window → WM_ACTIVATE →
CheckAndHandleBackgroundFullscreenOnMonitor() dereferences the dangling
HWNDMessageHandler*.