a customer notified me about difficulties resizing SL Windows.
I was able to reproduce this on your Test application.
It is a bit hard to explain so I prepared a video about it.
The video shows a red circle when I click with the mouse (unfortunately it does not show releases).
First a short description. I click on a corner - drag a little an finally my mouse "slips into the content area".
-- It also works on the "outside area" - but content happens more often (or on making the window smaller).
The corner still thinks the mouse is dragging - so when I (mouse button released) hit the corner it resizes.
To stop this you have to position your mouse on the corner and click (press / release).
As you can see I do not "sprint" with the mouse - I move it real slowly.
Anyhow - the problem becomes worse after some of this "mouse lost events".
The resize functionality gets lost on this corner (or the whole side).
And last not least (I don't know why) the corner becomes some kind of "run away thing".
When you touch it with the mouse - no matter from which direction - the window becomes larger.
In the last seconds of the video I tried to make a trick - I touch the corner from the outside (right bottom).
This was "last hope" that the corner will run away to top, left - but as written before - it does not matter from where you touch.
The window becomes larger and larger.