This is a known issue (see #1845, #1916, #1923, #1938, #2123, etc.) and is happening due to CSS defined elsewhere in your jupyterlab frontend, either in a labextension or the cell output of a notebook in the current session.

To address this, follow the steps here: #2123 (comment)

I have never been able to reproduce this issue, and so have not been able to identify the root cause or work on a fix. If you're able to use your browser tools to find the source of the offending CSS, please let me know!

Thanks for the solution! And apologies that I missed the known issue. I wasn't able to find the exact CSS via browser tools, but one thing I have in common with others who have experienced this problem is the use of Jupyter Widgets. In particular, I was using the following in another instance of Jupyter Lab.

    options_layout = ipywidgets.Layout(
        overflow='auto',
        border='1px solid black',
        width='300px',
        height='300px',
        flex_flow='column',
        display='flex'
    )

The options menu using flexbox as well, so perhaps this is related?

Nice - thanks for the clue! I'll see if I can reproduce it from that.