clicking window title bar and then quickly pressing LeftAlt+Space makes xfwm4 crash

Clicking the window title bar and then quickly pressing LeftAlt + Space makes xfwm4 crash. As a result, you temporarily lose window decorations and the title bar disappears until xfwm4 is automatically restarted after the crash.

Important: This bug does not occur if you click the window title bar and then wait 5 seconds before pressing LeftAlt + Space. So the bug seems related to event handling.

Steps to reproduce:

1. Left-click the window title bar of any application window like xfce4-terminal or galculator.
2. Quickly press LeftAlt + Space right after clicking the window title bar. We want to open the window context menu.
3. Observe that xfwm4 crashes.

Version info:

$ rpm -q xfwm4
xfwm4-4.20.0-3.fc43.x86_64