I bisected and found that this patch caused a regression in header-line formatting. I don't have the exact reproduction steps yet, but the gist is that I use a header-line with multiple faces in one window, then open a new popup window that has no header-line. The original window's header-line will get its faces changed slightly, but it's hard to tell how.

Here's a video demonstrating it: https://share.cleanshot.com/CD9PtVv3

You can see that the padding on the left of the header line goes away and the line numbers change to a variable pitch font. If I adjust my font size, the header line redraws with the proper faces. This will happen randomly, but I can force it with the font size change reliably.

I'll see if I can narrow reproduction steps, but it may be worth considering a revert for now.

Thanks,

Aaron