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I have absolutely no idea.
It probably seems "clean" and "fresh" to some prepubescent wit that works in the "design department" at MS. Like many Win 8 / 10 changes, "usability" was not considered or tested.
"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony
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Pretty sure this is one of those settings that get reset with every Windows update, 'cuz I've definitely used this before (months ago) and--now that you've posted about it--noticed that it was turned off again...
Perhaps Windows version difference or update history can explain this. FYI, the Windows version on my system is Win 10 x64 Pro, and the latest "Feature update to Windows 10, version 1903" was installed on 7th of August, 2019. There were also several other updates in the last 3 months … With everything up to date there should be no difference I guess. … And, while typing, I asked Google about this issue, and I got a better clue ... windows 10 - Can't find "Automatically hide scroll bars" option - Super User