Interesting... To draw 5px INSIDE then it seems I have to copy the splineset and move each point 2.5px inside the old shape. I'll have a further experiment tomorrow. Currently trying to move x/y and then determine if that point is INSIDE the shape. The INSIDE test is done using a ray from the new point and counting even/odd collisions with the splines in the splineset. (odd for rays in each direction == inside of shape).

But its not moving the points uniformly yet.

Doesn't Cairo have a method to paint inside an area?

See Preferences / Editing / OutlineThickness (set > 1 to have effect, eg 5).
Colour is from X Resource Editor / Outline View 2 / Active Thick Layer Color (default is grey with alpha so you can still see the normal outline also to verify things are as expected).

Setting outlinethickness needs a forced redraw to have effect, eg, by left clicking on the charview.

Nice. I've set the outline to 2, and then set the colours of 'Active Thick Layer Color' and 'Active Layer Color' to be identical (mid gray). Functions well imo.

cc @pathumego