Windows blur effects + Alternatetab = Flickering background

[van-de-bugger]

Fedora 29, gnome-shell-3.30.1-2.fc29.x86_64, xorg-x11-drv-nouveau-1.0.15-6.fc29.x86_64, $XDG_SESSION_TYPE == "x11", all (but two) gnome shell extensions are disabled, only "Alternatetab" v39 and "Windows blur effects" v12 are enabled. Settings of "Alternatetab" do not matter, Settings of "Windows blur effects" probably do not matter too.

Make sure one window is maximized (let it be Firefox), another window is not maximized (let it be gnome-terminal). Minimize or close all other windows. To reproduce the problem: Make Firefox an active window. Now press (and keep pressed) Super (aka Win), then press and release Tab (probably, several times) to switch to Terminal, make some delay (e. g. 1 second or more), and only then release Super. Terminal window appears first, then the part of the desktop (which is not covered by the Terminal window) is filled with the background (aka wallpaper) for a moment (flicks), then part of the desktop (which is not covered by the Terminal window) is filled with Firefox. Such desktop flickering does not affect functionality but very distracting and annoying.

Firefox and Terminal are not really important. It could be any two windows: one should be maximized, another should not.

Pause between releasing Tab and releasing Super is important. If Tab and Super are released quickly, so list of windows does not appear, desktop does not flick.

Other windows are not very important. If there are other non-minimized windows, they could appear for a moment instead of desktop wallpapers. However, if maximized windows look similarly (e. g. black letters on white background), flickering is a bit less noticeable.

Both "Alternatetab" and "Windows blur effects" are important. If any of these extensions is disabled, the problem does not appear.

Since I does not know if "Windows blur effect" or "Alternatetab" causes this flickering, I have submitted the ticket to both. See this ticket in Alternatetab bug tracker.