This was originally going to be an email to the mono winforms list, but it got too long, and people won't read long emails, so I'm putting it here instead. The following really applies to people working inside the mono winforms assembly, not consumers of it, but someone may find it interesting.
Recently I made a pretty fundamental change to the way SetBoundsCore (and bounds setting in general). Because it seemed to go very smoothly and only broke a few easily fixable cases, I didn't really alert people the way I should have. Everaldo found a case I missed, and there's probably more, so it's important that everyone knows what the change was and how it may affect them.
SetBoundsCore looks like this:
void SetBoundsCore(int x, int y, int width, int height, BoundsSpecified specified)
where BoundsSpecified contains things like X, Y, Location, Width, Height, Size, All, None.
The old understanding of how this worked, and the way mono implemented it, was that the BoundsSpecified indicated what fields you actually wanted to change, and the rest of the fields would be kept the same for you. Meaning you could call it like:
SetBoundsCore (0, 0, 0, 25, BoundsSpecified.Height)
and it would only change the height for you. It turns out to be quite different than this, as seen here.
The BoundsSpecified parameter is actually indicating which of the parameters has been explicitly set by developer code. You may note that if you have a button on a form, which you then dock, when you undock it, it snaps back to its pre-dock location and size. This magic is accomplished through the BoundsSpecified.
One place where this was a problem was DefaultLayout, which was setting things using the control's Location and Size property setters. This needed to change to a SetBoundsCore call with BoundsSpecified.None, which basically made the needed bounds changes, but the bounds code knows it was not explicitly set by a user.
I doubt I explained that very well, but here's the summary. Whenever you are moving or sizing controls inside winforms, there is very little chance that you want to do so using the Bounds, Location, Left, Top, Size, Height, or Width properties. If you do, the bounds code thinks it was explicitly set by a developer/user. What you really want to do is call SetBounds with BoundsSpecified.None. For example, to change the height of something, do this:
SetBounds(Bounds.X, Bounds.Y, Bounds.Width, 25, BoundsSpecified.Height);