I can’t seem to copy the geometry of one mask and apply it to another mask without them not matching. The target mask is either placed incorrectly on the stage, or it’s size is inconsistent with the source.
Intuitively, I would expect that whatever geometries are applied to the source mask should be copied to the target mask – width, height, and position.
I might redact what I just said 
It wasn’t working – and now it is working – doh!!
Well, I DID say it wasn’t “consistent”!!
Is there a way to reproduce this misbehavior?
No, not consistently – NOT every time.
I created a new slideshow and imported a bunch of images – animation disabled.
I went to a random slide, clicked on image, and created a rectangular mask using the Options palette to the right. The image was vertical, but the new mask was horizontal, and obviously didn’t match. I clicked back on the layer, copied the geometry, clicked to enable the mask, and then did a paste geometry, but the geometry of the image did not paste onto the mask (I ended up with a vertical image and a horizontal mask).
Sometimes I create a new mask (rectangle) and it doesn’t show up – I can see in the Mask pop-up that it is 0 x 0 pixels. Then I have to enter “something” big enough in the edit boxes so that the mask is visible and I can “grab” it with the mouse. I can again copy the geometry of the layer and try to paste it into the mask, but it doesn’t ALWAYS carry through.
So, the behavior IS reproducible, but not consistently (which makes it hard to debug).
Thanks for the detailed description. We will check the code where this mask size is determined to see if there is any flaw.