In Mac LR CC Develop screen small white rectangular box upper right hand corner..... what is it for?

"Does not solve the problem. Any other ideas? Is this a bug that Adobe is addressing or intends to address?"

Do the menu command Help > System Info. If that shows you are on LR 6, then no, Adobe won't address it. They stopped supporting LR 6 when they released LR 7 3.5 years ago.  See this reply for a workaround that has helped others:

https://community.adobe.com/t5/lightroom-classic/in-mac-lr-cc-develop-screen-small-white-rectangular-box-upper-right-hand-corner-what-is-it-for/m-p/11645625#M214634

If you're on LR 10.2 (the most recent version), then post a screenshot of what you see.

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I updated to LrC-release 10 and it appears to have gone away.  Thanks for the advice. - Mark

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 have the exact same issue.  Same size, same spot. Doesn't seem to respond to the suggested fix.

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'm guessing this is a malfunction of the graphics card/graphics driver.