The window manager (fork) i3-gaps (4.16.1) e-build no longer builds. I updated it manually to 4.21.1 (upstream) version, which fixed the issue.

Update: Perhaps we should remove i3-gaps from the tree. With i3 4.22, both i3-gaps and i3 will be in the i3 package. Since the latter is an autogen, we will get this feature in Funtoo.