Tray Settings
[X] Always show mIRC icon in tray
[X] Place mIRC in tray when minimized
[X] Single click on tray icon to open

With above settings you are able to click the tray icon to alternate between hiding and showing the main mIRC window (minimize and restore). If any window is set on top either a mIRC related window or a window from another program, you cannot click the tray icon to hide (minimize) the main mIRC window. I sometimes click the tray icon twice to briefly show and hide again, but cannot when *any window* is set to be on top. Thanks.

Confirmed, the window need to be shown though, using /window -do @test, if @test is minimized, it's working as expected.

Thanks for your bug report. This issue has been fixed for the next version.