tmux 3.0a contains a change to the swap-window command which alters its behaviour in focal compared to previous releases - see https://github.com/tmux/tmux/commit/39c55d5b6fcafd9476528ebc1064192c90f559a4 for more context.
The consequence of this is that the "move window" shortcuts in byobu (CTRL+SHIFT+F3/F4) no longer behave as expected in focal compared to previous releases. Using those shortcuts swaps the windows, but focus doesn't move to the originally focused window. Eg, if I have 2 windows (A and B, in that order) with A being the currently focused window, and then I use CTRL+SHIFT+F4 to swap them, I end up with windows B and A in that order (as expected), but window B is focused when I expected the focus to follow window A (as it did with the tmux version in eoan and previous releases).
This bug was fixed in the package byobu - 5.133-0ubuntu1
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byobu (5.133-0ubuntu1) focal; urgency=medium
* debian/docs: Closes: #951455
- fix FTBFS on README
-- Dustin Kirkland <email address hidden> Mon, 17 Feb 2020 08:11:50 -0600