Hey Andy and company,
I've found a bug in the ruler stencil functionality.
If the drawing tool you're using is wider than the stencil, the ruler will allow color from the tool to bleed through to the other side.
1) Set the marker tool size to 10%
2) In an empty layer, draw a line.
3) Make a stencil from this layer, convert it to a ruler, and invert it.
(You've now got a very thin ruler stencil covering the line you drew.)
4) Set the marker size to 100%, change the color, and move to a new layer.
5) Start on one side of the stencil, draw toward and past it, then along the length of the stencil and back to the original side.
You'll find that half of the width of marker minus the width of the stencil has drawn on the wrong side. I'm pretty sure you've got math that blocks (toolWidth / 2 - stencilWidth). That doesn't work when the stencil becomes thinner than the brush.
Note that you can repro this with the builtin measuring Ruler and the marker set to 500% size.
This bug blocks the ability to do easy fills. If this bug were fixed, it would be easy to draw a box, make a ruler stencil from it, and then use a big tool to fill the box in.
Now that I know what's happening, I can concievably do fills if I'm careful to use tool sizes that are less than twice the width of the lines I'm filling in. The trouble is, I tend to draw with thin lines, which means I'd need to use thin tools to color. It would be great to be able to set the tools large and slop color inside the stencil.