Neovim Cursor Flickers from Block to Bar running in MS Terminal under Tmux

[mebaran]

Environment

Windows build number: 10.0.19041.172
Windows Terminal version (if applicable):  0.10.781.0

Other software:
Debian WSL2 running tmux

Steps to reproduce

1. Start tmux
2. Start neovim (nvim -u NORC)
3. Enter insert mode
4. Watch cursor flicker between block and thin line

Expected behavior

Cursor should remain a line and not flicker. (this is the behavior in WSLtty)

Actual behavior

Cursor flickers between block and line.

[DHowett-MSFT]

This doesn't seem to reproduce with stock configs of tmux and neovim on Debian:

no cursor flickering here.

[mebaran]

Thank you for investigating this. To reproduce please run the following before to recreate the cursor flicker:

tmux set -g status-right %H:%M:%S
tmux set -g status-interval 1

That seems to cause the cursor flicker issues. The flicker seems to occur in tandem with tmux rewriting the status bar (if you set the interval to 5, the cursor will flicker every 5s). I can confirm the same issue doesn't happen in a mintty based terminal.

[DHowett-MSFT]

You know what, this looks like it's probably because we typoed the escape sequence that turns off the cursor. tmux and vim "fight" over the cursor, but tmux turns it off before it writes out the status line.

This is incidentally fixed by #5181 (ref #3093)