I originally took the question to be about
tmux but it was apparently about
screen. Edits to the question showed this to be the situation later on. I'm leaving this here in case someone else has this same question/issue with
Based on what you're showing it sounds like your split pane borders are being shown as dashes. You likely did this with a Ctrl+b+".
If you use Ctrl+b+n you can switch to the next pane and attempt to close it by typing
exit or Ctrl+d.
What about the dashes
The dashes appear to be due to this option in your terminal. In iterm2 on OSX, for example, you can go into this menu and disable this rendering: Preferences → Profiles → Text and change the setting of "Treat ambiguous-width characters as double-width."
Once you've got the settings in place, the dashes should look like a solid line:
If you find the above is not your issue, it may in fact be due to the font you're using. Your font just may not have an appropriate glyph, and so the dashes are what get used.
If you feel this is your issue, take a look at this SO Q&A titled: Tmux borders are drawn with dashed lines; how can I change them to continuous lines?.