Full screen apps turn grey.

easy repro:
I was trying to work around https://github.com/emersion/slurp/issues/43
But this will happen in any fullscreen game, if a game can be set to "borderless windowed" it seems to work, but imv seems to invoke "true full screen" with or without the -f flag, and breaks this script from being useful:

```
OUTPUT=$(swaymsg -t get_outputs | jq -r '.[] | select(.focused).name')
FOCUS=$(swaymsg -t get_tree | jq -r 'recurse(.nodes[]?, .floating_nodes[]?) | select(.focused).id')
grim -t ppm -o $OUTPUT | imv -f -w screen-shot - &
IMV_PID=$!
grimshot --notify copy area
imv-msg $IMV_PID=$! quit
swaymsg "[con_id = $FOCUS] focus"
```

Start by reproducing the provided shell script with a true-fullscreen game or imv, comparing it with borderless-windowed mode. Check how slurp behaves alongside swaymsg, grim, and imv, and define done as keeping the region-selection workflow usable without turning fullscreen applications grey.

Written by the indexing model from the issue text.