Unusual "half border" situation with tmux

My tmux occasionally gets into a weird state where half the border (regardless of orientation, or (tmux) window focus) is bolded, highlighted?

See the screenshot below:

Weirdly, even after completely killing tmux I can recover from this state, but then half the border is colored, see the second screenshot:

In the second screenshot, although subtle half the border is colored white, the other half green.

Note: The orange/green difference is irrelevant here, I tried a "fix" for weird borders which forced them to orange, and that affected the screenshot I made.

I don't believe I have any unusual configuration:

new-session -n $HOST
set -g @plugin 'tmux-plugins/tpm'
set -g @plugin 'tmux-plugins/tmux-sensible'
set -g @plugin 'nhdaly/tmux-scroll-copy-mode'
set -g @plugin 'tmux-plugins/tmux-yank'
set-window-option -g mode-keys vi
set-window-option -g allow-rename off
set-window-option -g automatic-rename off
set-option -g mouse on
set -g @shell_mode 'vi'
set -g @yank_selection 'clipboard'
set -g @yank_selection_mouse 'clipboard'
set -g default-terminal "screen-256color"

Is this a mis-feature, why is only half colored/bolded, why does the bolding only happen sometimes, and why is it not affected by zooming or mouse clicking in or around the windows?

The fact that I cannot reliably reproduce, nor dismiss this once it happens leads me to assume this is a bug but I'm just not sure.

tmux version is 2.9x, terminal is alacritty 0.4.0, operating system is Feroda, and my $TERM variable seems to be screen

Solution 1:

This is normal.

The active pane is embraced in a colored frame. When there are exactly two panes, just coloring the whole dividing line would say nothing about which pane is active. For this reason tmux applies the color to one half of the line. If you make the other pane active, the coloring will change. This way you can tell which pane is active.

"Bold" line behaves in the same manner, only it indicates the marked pane (default keystrokes to mark/unmark: prefixm or RMB).

In your first screenshot the left pane is marked but the right one is active. In your second screenshot there is no marked pane and the bottom pane is active.