[Firefox on Wayland] When Firefox is switched by Alt+Tab, ALT remains pressed for mouse scroll events

When Firefox is switched by Alt+Tab, ALT remains pressed for mouse scroll event. That causes regression on background scrolling when history is scrolled instead of content (https://bugzilla.redhat.com/show_bug.cgi?id=1650051)

There's a minimal testcase attached:

test6.c

Reproduction steps:

1. Build
2. Run some application, terminal for instance
3. Run testcase under wayland, make it focused
4. Scroll by mouse wheel, see the printed modifiers (0x10 in my case)
5. Change focus to the app from 2) by ALT+TAB
6. Go by mouse cursor on inactive/unfocused area of the testcase, scroll by mouse wheel, see 0x18 modifiers printed (ALT key pressed)
7. Repeat the step 5) by mouse click
8. Repeat step 6, see only 0x10 modifier printed (without ALT)