Stacking windows on each other and then unstacking produces a glitch

[rudzik8]

Tested on: hrev56640 x86_64 (real hardware, running from USB)

When you stack a window on a window (doesn't matter which ones: resizeable or not, bigger or smaller, etc), and then unstack the second window out of the group, it produces a glitch. Tab of the second window (which was just unstacked) stays here but as a shadow on user's desktop: when you try to hover your cursor on it or select it by dragging the blue selection thing on it it just disappears. Also, the unstacked window moves weirdly, not like the user moves it by its' (now) single tab, but like the user moves it... by air?!

Screenshot of it (in process of reproducing):

[humdinger]

A duplicate of #16107.