I'm using i3 version 4.16.1 on Manjaro XFCE with XCompMGR as the compositor.
Why does this happens when I resize a window in i3? It doesn't seem to be a compositing issue since this appears even with the compositor disabled. Could the window be updating too fast? Or too slowly? I don't know.
By the way, this never happened on the default XFCE interface.
I tried asking this on the i3wm subreddit, but apparently people there don't know either.