Thank you so much, Petyo, for thorough reply.
If I had known about this before, I'd have implemented my site with with react-virtuoso (as IE11 isn't of a concern).
The variable size content workaround in my implementation was not as performant as I tried to re-calculate the content size depending on the current viewport.
And the jankiest part was the keyboard navigation (as you graciously pointed out the cause of the issue.)
As those (batteries) are included, I will look in the future site to use react-virtuoso (unless I get time to go back to the current site to replace the react-window version with).
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