Hello there, I spent some time trying your solution and finally found working one.
If AutoScrollMargin is 1,1 or any other size except exact size of ScrollBar (or multiply of it) I see same issues as before and even more on High DPI. I attach project without the fix with AutoScrollMargin = 1,1.
If you click on label on 100% DPI, you will see the issue, I believe it yours, not Microsoft's. Form will shrink, you will be able to scroll txt with mouse, scrollbar will have Visible=true, but won't be actually displayed.
First issue happens when height is constant, width is big enough to avoid scrolling, and then you step by step reducing ONLY width with small steps (5px).
On HDPI the project will finally hang in cyclic resize events.
Hanging happens when scrollbar need to be shown or hidden, so you need to jump over that critical size in both directions.
Also on HDPI scrollbar will be show only when two lines of text are hidden, if only one line of text is hidden - scrollbar not shown.
For reproduce you need to set size big enough to avoid scrollbars, and start reducing height with small steps.
If I set AutoScrollMargin to size of scrollbar or 2x, 3x of it, it will work almost perfect. If it is at least one pixel bigger or smaller - all issues comes back, just in different size.
I fell like it can be big issue overall, since it uses some basic code for calculating sizes, and can be used as internal component of other controls. I tried to subscribe to all possible resize events of ScrollablePanel and ScrollablePanelContainter, it doesn't solve issues. So at the moment the problem has workaround, but please consider some more time in deeper looking into it.