change overlapping window update region

[todma]

Hello!
I have 2 windows, A and B.
A overlapps B. (is in front of)
Normally, if A moves, the contents of A are not updated.  B, being
behind A, is updated to reflect A's motion over B.....
Also, if B is updated, A is not updated, as it's on top of B.
All this to reduce flicker.

I'd like to change this; if A is moved, the contents of B that are
under A is updated.
If B changes, also the area of A is included in the update.

This is to implement a transparent A that shows B underneath it.

(sort of using a window or dialog A like a text sprite with
transparency)

Any advice on how to do this? (I know i should know how to do this
already!)
thanks!
Todd.