Try setting MigLayout("insets 0");
EDIT:
To fix for gaps (see comments) you can simply write: MigLayout("gap 0");
Or, gapx and gapy.
All this comes from: http://migcalendar.com/miglayout/cheatsheet.html
Question
Is there a way to make the default gap between components of all layouts ever to be created in a specific program to be 0?
Is there any way to do it? If not, is there at least a way to do it on a layout-level (new MigLayout("gaptozero");)?
Solution
Try setting MigLayout("insets 0");
EDIT:
To fix for gaps (see comments) you can simply write: MigLayout("gap 0");
Or, gapx and gapy.
All this comes from: http://migcalendar.com/miglayout/cheatsheet.html