I purposefully froze my terminal with CTRLS in visual studio code, so that I could inspect the fast log messages scrolling through. Normally, you can unfreeze it again with CTRLQ.
However, it seems that vscode catches this second shortcut and instead of unfreezing my terminal, it opens this popup in the middle of the screen - which disappears as soon as I release the keys...
This seems to be called the "Quick Open View".
I assume that there is a way to send the "unfreeze" signal to the terminal. How?
My solution for now was to detach tmux in vscode, then connect over ssh using a normal terminal, attach tmux, send the CTRLQ there, detach and exit again, and re-attach in vscode.