PC3 menu: make interactive condition more robust
If the menu has a content height that exceeds the available height, but ApplicationWindow.window isn't set--for example because we're in the SDDM login screen where there is no application window--then we'll hit the fallback condition and be non-interactive, which makes the menu terribly difficult to scroll with a pointing device and impossible to scroll with a touchscreen.
To fix this, consult Window.window rather than ApplicationWindow.window. This is what other styles do.
BUG: 466968 FIXED-IN: 5.105
(cherry picked from commit e4830f16)