The designer is implemented with his own drawing of the gadget and has nothing to do with the current OS. So you can use the designer to draw different OS. However, the minimum sizes of the different OS are not considered.
It is well known that sometimes under Linux with GTK3 the gadgets don't fit that well.
Take a look at the module ScaleGadgets. With these you can do an automatic size adjustment.
But this is still limited by Linux with the minimum sizes.
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