Shrinking list box

Never seen this before.
I have two option buttons, if I togle between the two, listbox1 height shrinks.
It will go from 108.7 to finally togling between 11.8 and 12.6.

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Edit 1:
I added code to force the height, but when togling you can actually see the height change.
Is this due to changing Fonts?

Edit 2:
I set the Integral height to False, this seems to maintain the height but I cant scroll to the bottom of my list when option button2 is true.
So I am back to square one. I removed the following code form both option buttons:

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Thus the listbox shrinks.

Any hints, tips or examples are appreciated.

FYI
By extending the listbox height beyond the list's length (Rows) the listbox quit shrinking.
I think this is an issue between Tahoma and IMS font sizes.
Font size 8 for Tahoma appears to be larger than font size 8 for IMS.

Does it make any difference if you change the IntegralHeight property.

I set the Integral height to False, this seems to maintain the height but I cant scroll to the bottom of my list when option button2 is true.

The only thing that prevents the shrinking is the list box height needs to be greater than the height of the rows for the list of items.
I figured this out with trial and error.

p.s.
This wasn't a problem when I was using Checkboxes. I switched to optionboxes because some one said it would be better.