Over on the Fediverse I shared a fun Firefox issue I've just run into :
Today's fun Firefox bug: if I move my (Nightly) Firefox window left and right across my X display, the text inside the window reflows to change its line wrapping back and forth. I have a HiDPI display with non-integer scaling and some other settings, so I'm assuming that Firefox is now suffering from rounding issues where the exact horizontal pixel position changes its idea of the CSS window width, triggering text reflows as it jumps back and forth by a CSS pixel.
(I've managed to reproduce this in a standard Nightly, although so far only with some of my settings.)
Close inspection says that this isn't quite what's happening, and the underlying problem is happening more often than I thought. What is actually happening is that as I move my Firefox window left and right, a thin vertical black line usually appears and disappears at the right edge of the window (past a scrollbar if there is one). Since I can see it on my HiDPI display, I suspect that this vertical line is at least two screen pixels wide. Under the right circumstances of window width, text size, and specific text content, this vertical black bar takes enough width away from the rest of the window to cause Firefox to re-flow and re-wrap text, creating easily visible changes as the window moves.
A variation of this happens when the vertical black bar isn't drawn but things on the right side of the toolbar and the URL bar area will shift left and right slightly as the window is moved horizontally. If the window is showing a scrollbar, the position of the scroll target in the scrollbar will move left and right, with the right side getting ever so slightly wider or returning back to being symmetrical. It's easiest to see this if I move the window sideways slowly, which is of course not something I do often (usually I move windows rapidly).
(This may be related to how X has a notion of sizing windows in non-pixel units if the window asks for it . Firefox in my configuration definitely asks for this; it asserts that it wants to be resized in units of 2 (display) pixels both horizontally and vertically. However, I can look at the state of a Firefox window in X and see that the window size in pixels doesn't change between the black bar appearing and disappearing.)
All of this is visible partly because under X and my window manager , windows can redisplay themselves even during an active move operation. If the window contents froze while I dragged windows around, I probably wouldn't have noticed this for some time. Text reflowing as I moved a Firefox window sideways created a quite attention-getting shimmer.
It's probably relevant that I need unusual HiDPI settings and I've also set Firefox's layout.css.devPixelsPerPx to 1.7 in about:config. That was part of why I initially assumed this was a scaling and rounding issue, and why I still suspect that area of Firefox a bit.
(I haven't filed this as a Firefox bug yet, partly because I just narrowed down what was happening in the process of writing this entry.)