Environment

[Environment]::OSVersion

Platform ServicePack Version      VersionString
-------- ----------- -------      -------------
 Win32NT             10.0.18363.0 Microsoft Windows NT 10.0.18363.0

Windows Terminal version: 1.6.10412.0

Ubuntu 20.040 in WSL2

Steps to reproduce

1. Install "DejaVu Sans Mono for Powerline" font in the system (for example from https://github.com/powerline/fonts/tree/master/DejaVuSansMono), or non-Powerline original DejaVu (it behaves just the same as its Powerline derivative)
2. Set it as a default font for Windows Terminal or just for WSL and of size 9 (that's the only size when issue manifests):

   "fontFace" : "DejaVu Sans Mono for Powerline",
   "fontSize" : 9,
   

3. Start Ubuntu in WSL tab
4. Start tmux
5. Split window into vertical panes:

   Ctrl+B
   :split-window -v
   Enter
   

Expected behavior

Thin line between panes.

The delimiter line (which is just a unicode symbol "─" (copy-pasted here from the terminal terminal) is displayed unless specified above combo of font & size are used.

I.e the following setup changes make the delimiter to appear:

1. The same font, but sizes != 8 (screenshot above is with size = 10)
2. Another font (tried "Cascadia Code PL", "Source Code Pro for Powerline", "Ubuntu Mono derivative Powerline", "Roboto Mono for Powerline")
3. Display scaling of 150% (on 100 & 125% the issue still happens), thus on HiDPI displays with enabled by default scaling you won't see it.

Actual behavior

No visible line between panes: