I'm guessing this is a malfunction of the graphics card/graphics driver.
Please try to switch off the GPU support from the Lightroom preferences and check if that helps to fix the issue.
Go to Lightroom > Preferences > Performance tab > Uncheck "Use Graphics Processor" > Restart Lightroom.
That you have a Graphics subsystem Problem. Video card and or driver problem.
As above in the nested reply by Axel Matt turn off the GPU optioon and Test.
I have the exact same issue. Same size, same spot. Doesn't seem to respond to the suggested fix.
Seems to coincide with installation of MacOS BigSur (11.0.1).
Lr Classic V6.0 (yes, it is old)
A few other bugs noticed in using. meh.
Others have reported glitchy white boxes at the top of Develop in LR 6 on Mac OS 10.11. A workaround that has helped others: Do View > Loupe Info > Show Info Overlay to turn off the info overlay. You don't get to see the info overlay, but you won't see the white box either.
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I updated to LrC-release 10 and it appears to have gone away. Thanks for the advice. - Mark