Right-clicking on a window titlebar normally offers the option to make
it "Always on top".  However, gnome-shell disables this menu option for
maximized and half-maximized windows.

If I un-maximize a window, mark it as always on top, and re-maximize (or
half-maximize) the window, it stays always on top, but the menu item
still becomes disabled (which then does not allow turning off
always-on-top).

This menu option should always remain enabled.

- Josh Triplett