Trouble-shooting non-resizable stack/card size creep?

[Keith Clarke]

Hi folks,
I have a non-resizable stack (1024 x 768) with a common background group for all cards, including a 1024 x 768 background image.

Suddenly, whilst adjusting control positions on one card, I noticed that aligning its loc to 512, 384 no longer centralised the control - and a border of 4 or 5 pixels had appeared at the side and bottom of the background image. 

However, all cards and the stack (and background image itself) all seem to be sized at 1024 x 768.

Any ideas where I should look for the anomaly?
Best,
Keith..