Missing key release events on winit backend results in permanently pressed modifiers

[sodiboo]

When running in the winit backend, we don't always get release events from the outer compositor/window manager.

You can reproduce this issue reliably like so:

1. Open niri nested in niri
   • i use niri myself, and tested this on niri. it might work just as well in other compositors
2. Focus nested niri
3. Press any modifier key, e.g. "Super" (and do not release)
   • nested niri gets a KeyState::Pressed event
4. Click with the mouse cursor on another window in the top-level compositor
5. Release the modifier key
   • nested niri gets no event
6. Focus nested niri again

Now, xkbcommon will be in an annoying state where it reports that modifier as being permanently pressed.

These faulty modifiers are observed by Smithay here:

smithay/src/input/keyboard/modifiers_state.rs

Lines 30 to 41 in 8287457

	impl ModifiersState {
	/// Update the modifiers state from an xkb state
	pub fn update_with(&mut self, state: &xkb::State) {
	self.ctrl = state.mod_name_is_active(&xkb::MOD_NAME_CTRL, xkb::STATE_MODS_EFFECTIVE);
	self.alt = state.mod_name_is_active(&xkb::MOD_NAME_ALT, xkb::STATE_MODS_EFFECTIVE);
	self.shift = state.mod_name_is_active(&xkb::MOD_NAME_SHIFT, xkb::STATE_MODS_EFFECTIVE);
	self.caps_lock = state.mod_name_is_active(&xkb::MOD_NAME_CAPS, xkb::STATE_MODS_EFFECTIVE);
	self.logo = state.mod_name_is_active(&xkb::MOD_NAME_LOGO, xkb::STATE_MODS_EFFECTIVE);
	self.num_lock = state.mod_name_is_active(&xkb::MOD_NAME_NUM, xkb::STATE_MODS_EFFECTIVE);
	self.serialized = serialize_modifiers(state);
	}
	}

I implemented a workaround on compositor side:

• Workaround winit modifiers issue YaLTeR/niri#249