This happens in the Power Query editor of the desktop. Has happened before, but seems more common on the April/May 2020 builds. Only fix is to close and reopen Power Query. Closing the Custom Column box does not fix it.
I used May version to test but could not reproduce the issue on my side. Have you checked the display setting?
My display settings are not the issue @v-yuta-msft - I've seen this happen on 3 different computers from different manufactures. It is hard to reproduce and I suspect something internal in how Power BI is rendering and running out of resources in some way. In this particular example the issue manifested itself within 15min of creating a new file and getting to work. I've seen it happen after being in the Power Query editor for many hours, and on most occassions, it never shows up in a work session.
But it does show up enough to be frustrating, and thus I thought I'd report it.