fix(xwm): Defer X11 focus release between same-client transitions

[mayakwd]

Description

X11Wm set _NET_ACTIVE_WINDOW = NONE for any FocusOut event on a tracked top-level window, ignoring detail field. It breaks X11 clients like Unity Editor.

Mechanism of the flaw:

• the client gets focus at the top level and...
• almost instantly switches focus to its sub-window.
• X-server generates FocusOut(detail=inferior) for the top-level window.
• code without fix just set _NET_ACTIVE_WINDOW to none, and...
• there is no one to receive FocusIn.

But that's not the whole issue. X11Surface also synchronously calls set_input_focus(NONE, NONE), producing the intermediate state "focus is nowhere".

Fixes:

• [bug] Unity Game Engine Editor - Choppy framerate in Game View and Scene View after pressing Play pop-os/cosmic-comp#2006,
• Unity (game engine) editor misbehaves on Cosmic pop-os/cosmic-epoch#2311
• and potentially many more issues overall for X11 apps related to focus out.

Works not only for cosmic-comp but also for other compositors based on Smithay, all of which suffer the same issue.

Symptoms

Symptoms are exactly as described in pop-os/cosmic-comp#2006:

• Open a project in Unity
• It's smooth and nice
• Press Play, and experience a framerate dropped to 10 frames per second (Unity turns on throttling when losing focus, to not waste resources).
• More than that, the keyboard and mouse are unresponsive inside Game view
• Alt+tab and back - helps to return focus, but framerate is still down.

And many other related to input-unresponsiveness and sluggish experience, which cannot always be resolved by clicking other panels/alt-tabbing.

Diagnostics

Instrumentation wired into Smithay showed that:

  02:06:02.408  X11Surface::enter                          WINDOW=29360433 MODEL=LocallyActive
  02:06:02.412  xwm Event::FocusIn   WINDOW=29360433 detail=NONLINEAR tracked=true
  02:06:02.412  xwm: _NET_ACTIVE_WINDOW = window           WINDOW=29360433 
  02:06:02.462  xwm Event::FocusOut  WINDOW=29360433 detail=INFERIOR tracked=true
  02:06:02.462  xwm: _NET_ACTIVE_WINDOW = NONE             WINDOW=29360433   <-- issue

Meaning that Unity switched focus from the top level to a sub-window, but _NET_ACTIVE_WINDOW became none.

How the fix works

• FocusOut now doesn't touch _NET_ACTIVE_WINDOW
• Losing focus now deferred to the next event loop, leaving a space to cancel focus leaving for X11Wm.

What I've checked

• Checked locally with cosmic-comp (no code was changed in cosmic-comp, only path to local version of smithay)
• Unity Editor is no longer throttling
• Switching between X11<->Wayland apps works as expected
• No regression issues found

Checklist

• I agree to smithay's Developer Certificate of Origin.