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.
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,