I can't for the life of me figure it out how to create properly functioning taskbar indicators (when apps are opened, hovered over etc.).

I looked at existing themes and couldn't reproduce the results. Even with a very simple test file it doesn't work.
This is the test file:

This is how it looks - you can see a very slight line above the pink (but maybe that's the taskbar edge?) but more importantly blue lines right and left (and also a small green line between blue and light blue):

I tried changing the sizes. I tried painting margins; it all looks fine there:

What am I doing wrong?