Scroll buttons in the docking window do not seem to work as intended

[MartinZaleta]

It seems that whenever I use a WinForms docking container with more docked windows than fit in the panel, I cannot use the scroll buttons on the right to get to them all. It only alternates between two wrong behaviors and it also seems completely random which one of these two will happen.

I start at the following state:

There are still two panels hidden on the right and I want to get to them, but alas, it only hides more of them:

Sometimes (no logic when) the right button works, but only for a while. There is still another hidden panel (even behind the partially visible one), but further clicks on the button do nothing at all:

Any ideas?