I could not find an answer to this issue on Windows when searching Stackoverflow/Google/Bing, and this thread was quite prominent hence my Windows-specific answer here.
CTRL+SHIFT+ENTER did not work for me. On windows 10 with Jupyter 4.4.0, Jupyter Lab 0.35.4, the tabs in Jupyter Lab can be made visible/hidden by clicking CTRL+SHIFT+D or toggling in the menu the options (View > Single-document mode).
Jupiter Lab, menu item for toggling the Single-document mode