Description

After installation, jupyterlab was normal for a while, but after a few key presses or clicks(I can't tell clearly) today, I sudenly found the file name tag on top went away, I tried to restore the layout, but failed.

Reproduce

I can't undo this, nor do this again.
Even after rerun jupyterlab, the missing header layout remained.

Expected behavior

I hope each file should have a tag, at the top of its file when editing.

Context

Welcome to Ubuntu 18.04.1 LTS (GNU/Linux 4.15.0-74-generic x86_64)
Browser: Chrome 79.0.3945.117;
$ jupyter-lab --version
0.27.0