Standalone (and stack in IDE) getting larger on each launch

[James Hale]

I have an app that stores the height of the main stack as a custom property. This main stack is called from a splash stack in the standalone.
During preopencard I check the property and set the size of the mainstack  accordingly.
If there is no cp I set it to a default value.
The user can change this and the app will store the new value.
This almost works in that the cp is set and if the size is changed the stack does remember there is a change.
However every time it opens it gets bigger by what I think is 22 pixels.
I am on a Mac.
What's weird is that if I check the cp value, it is as expected.
If I check the stack height (in the property inspector,) it is as expected.
It is the bottom of the stack that moves, all controls staying correct in their relationship to the top of the window.
Now interestingly, if I use the resizing function to set the height of the stack to what I thought it should be, and what the cp and the property inspector of the stack (height) tells me it is, the stack shrinks back to what it should be and the stack height as displayed in the property inspector remains the same!

Can anyone suggest what might be happening?
Given the increase looks suspiciously like the menu at height could this be playing a part? I have discounted that as only the bottom of the stack seems to move. All controls stay put.

James