I just saw this <> then read this and did some tests.

The doc say: “The distance (in pixels) of the caption from the left of the button.”
Yes that happens if CaptionAlign is Flush Left, but not if it is Flush Right (or Center). If I have Flush Right the distance is from the right of the button.

For the case, my guess is that they do some calculation to change the Caption to Capt… if it doesn’t fit on the button. Maybe they start at 1 value instead of 0, that’s why the workaround, changing CaptionDelta from 0 to 1, fix the issue.

If I have Left with CaptionDelta -20 the Caption doesn’t change to …ption and I wonder why if I select Right and it doesn’t fit (I know the Case is a bug) it change the last few letters to dots? Is that a standard UI thing? Why don’t just put what it fits?