Bug: Maximized always-on-top window is incorrectly stacked below another monitor's window

Distribution

Mint 21.3

Package version

6.0.5

Graphics hardware in use

Radeon RX 5600 OEM/5600 XT / 5700/5700 XT

Frequency

Always

Bug description

On Linux Mint 21.3 (Virginia), Cinnamon with Muffin 6.0.1, a maximized Firefox window with _NET_WM_STATE_ABOVE can be incorrectly covered by a PhpStorm window on another monitor when that window receives focus.

The _NET_WM_STATE_ABOVE property remains set, but Muffin nevertheless places the PhpStorm window above Firefox in the stacking order.

The bug does not occur if _NET_WM_STATE_ABOVE is enabled while Firefox is not maximized and Firefox is maximized afterwards.

Steps to reproduce

• Open Firefox on one monitor and maximize it.
• Enable "Always on Top".
• Open PhpStorm with windows on both monitors.
• Focus a PhpStorm window on the other monitor.
• Observe that PhpStorm is stacked above the Firefox window despite Firefox having _NET_WM_STATE_ABOVE.

Expected behavior

Always on top

Additional information

No response

Start by reproducing the issue on Linux Mint 21.3 with Muffin 6.0.1 using the listed Firefox and PhpStorm multi-monitor steps. Trace the relevant window-stacking behavior in libmuffin and the sample WM binary, muffin, then verify that a maximized _NET_WM_STATE_ABOVE window remains above focused windows on another monitor.