Incorrect per monitor filtering of borderless windows in "Window Buttons"

Steps to reproduce:

1. Uncheck Show windows from all monitors in Window Buttons, so that only windows from the current monitor are displayed
2. Ensure that the Window Manager > Keyboard > Fill Window shortcut is set
3. Open Chromium (or any other borderless application) and use the Fill Window shortcut (not maximize!). If separate monitors share a side border, both will display Chromium in their respective Window Buttons elements. However, if Use system title bar and borders is enabled in chrome://settings/appearance, the window is correctly considered to be on a single monitor (after application restart). The same correct behavior occurs when you maximize the window, resulting in a visually identical appearance to using the Fill Window shortcut.

This may be an issue with libxfce4windowing, I’m not sure.

xfce4-panel 4.20.3 (Xfce 4.20)