Moving the mouse to a different output does not always grey out rectangles

[Ragnar Groot Koerkamp]

When using sway with two monitors, with one or more windows active on both monitors, the following bug occurs:

• Have one (or more) windows open on each monitor
• Run grimshot save window, which calls slurp -r and passes in the rectangles for all windows.
• Moving the mouse (quickly) back and forth between the two outputs occasionally (~50% of times) does not blur/gray out windows.

Expected behavior: When the mouse leaves a window, it is always grayed out
Actual behavior: When the mouse leaves a window (and moves to another output), the window is sometimes not grayed out.

Note 1: everything works fine when quickly moving the mouse between windows on the same output.
Note 2: This happens both with and without using gaps in sway. (I.e. it doesn't matter whether the two rectangles exactly touch or not.)