I realize this is a very immaterial issue, but figured I'd open it anyway.
When opening foot on Sway, it seems like the first frame is drawn very small, and then by the next frame the window is drawn at the correct size. This causes the display to "flash" when opening foot. I've included screenshots below.
This may be a sway issue rather than a foot issue, but I haven't seen this in other applications on sway. I'm running Sway 1.7 and I see this behaviour with the latest foot commit 4bb3b538.
The first screenshot is the first drawn frame after starting foot. The second image is how the window "settles" a split second later.