Inconsistent size/position handling for overlays

[toaster]

The size and position handling of overlays is inconsistent and breaks things or causes dirty hacks.
An overlay is supposed to always have its canvas' size. That's why a canvas resizes its overlay to its size if it gets resized. Thus giving an overlay a different size doesn't make sense because it will be resized on a window resize.
The PopUp is designed to be an overlay which behaves like normal content (settable size and position) but under the hood it is an overlay and tries to satisfy the requirement of pos(0,0) and canvas size.
That's why PopUp.Size() doesn't report what PopUp.Resize() sets. But the latter must not be used with the canvas size because it affects the PopUp's content.
The same is true for the PopUp's position.
Because of this there is PopUp specific code in the canvas which is obviously bad.

This whole issue also is the root cause of #706.

I propose to wrap overlay objects into an special (probably internal) structure. This should handle the size/position correctly while the size/position handling of the overlay's content works normally.
The details still need to be carved out but this would remove the cause of #706 and the special code in the canvas. It also will make the fix of #706 obsolete if one has been put in place.

[andydotxyz]

is this resolved now @toaster ?

[toaster]

No, no work has been done on this.