Thank you White Tie. I don't think that it's down to the 72.000 resolution (I tried to change it using The GIMP and it changed on-screen, but when Exported as a .png ifle and reloaded it was back to 72.009.
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Ok, forget the button in my own theme for the moment. I tried a couple of things with the v4 Default theme.
I took the 2 Snap toolbar buttons and the Undo toolbar button and added a coloured boundary to each one. As I used The GIMP, the revised .png files now have a 72.009 resolution.
This first pic shows them appearing as intended in the Main toolbar. However in the MIDI Editor toolbar, the Snap button is OK (I used a yellow boundary for the Snap Off image) but the Undo button is not - the 30th row of pixels is missing and it is a different colour (overlaid by the composite_overlay image?).
As an experiment, I then used the Snap image for the Grid button - you will see that it is 1 pixel low, the 30th row of pixels is missing and it is a different colour. So, with the same image file, I get 2 different appearances.
In this pic, I switched on Snap - so the Snap_On image is used (I used a green boundary colour) and that is as expected. I also switched on the Grid button - it is still 1 pixel low, the 30th row of pixels is missing, and the boundary is not green (I think it is using the third image from the Snap Off png file).
I can't quite articulate what's going on, but it's all a bit funky.
Attached are the three modified buttons, for experimentation.