My VSCode instance has been refusing to show more than a single tab for a while, even with preview disabled and when double clicking on a filename to open.
Most blog posts talking about how to fix this have a similar resolution to this Stack Overflow thread. That is:
This didn't work for me, nor did setting
What worked for me is to lock a tab as opened by pressing
Ctrl+K+Enter whilst the tab is focused. This updated the UI to show a tab bar and when I double clicked on another file it opened as expected.