Fast switcher leaves windows translucent if they're removed while it's open

Describe the bug
Windows which are hidden or removed from the toplevel view list between keypresses of Fast Switcher, while the modifier key is still held, stay translucent when they minimize themselves on switch. This affects, for instance, Minecraft.

To Reproduce
Steps to reproduce the behavior:

1. Enable Fast Switcher
2. Launch a game of Minecraft
3. Fast Switch to another window
4. Minecraft is now stuck translucent until quit

Expected behavior
Windows which have been removed from the list should have their alpha level restored.

Screenshots or stacktrace
I may supply a screen shot of this.

Wayfire version
0.8.0, track-wlroots as well.

This may be difficult to fix, since the plugin just polls for the whole visible window list on every keypress, rather than maintaining a persistent copy of the list. It would need to diff the lists on update and fix any windows which have dropped out of the list, but haven't been destroyed yet.

Start by locating the Fast Switcher plugin and the code that polls the visible window list on each keypress. Reproduce the Minecraft case, then inspect how alpha is restored when switching or minimizing windows. Done means windows removed from the list while the modifier is held no longer remain translucent.