A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
I didn't notice that when I was using 4K, but I have to admit I didn't specifically look for it then (I don't use fill handle very much).
This is not something MS will bother fixing in 2010. But I suspect you'll find the same thing in 2019 (or even worse!).
Sorry to say, there is no control in Excel or Windows that will fix that problem.
Have you tried looking at the spreadsheet in Excel Online. It may be slightly "better".
Office Online (Propaganda page)
This page is one way of accessing Office online. Just sign in, then click on the "Get started for free" button to take to a home page with buttons for all of the Office Online applicastions you have access to.
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You should submit this complaint as a "Feedback" to MS. It is something they should work on since 4K monitors are coming to be much more common.
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Send Feedback to MS Developers ****Launch Feedback Hub via <WIN><F>
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