Previously, we triggered a layout (and hence a repaint) when sliding the
draw because we gave the draw loose constraints. The drawer uses an
Align to move itself to the proper side of the screen, so it can have
tight constraints, which makes it a layout boundary.

Also, don't trigger a layout just because the Scaffold rebuilds. There
isn't any state in the scaffold custom layout, so it doesn't need to
repaint just because we created a new instance of the delegate.

Finally, add the debugging infrastructure I used to find these issues.