VS 2010 Painting When To Draw/erase Snap Lines?

I have started to work some more on my 'shape editor'. If you're not familiar with it,ust imagine the Visual Studio form designer: a 'canvas' (the form) with a bunch of 'shapes' (the controls) that can be selected, moved, resized, etc. I have just finished implementing snapping between shapes and the canvas boundaries. But now I am having some slight issues with painting the snap lines.Just FYI: this is what I mean by snap lines:The blue lines that indicate that the 'buttons' are snapped to each other, and a purple line indicating that button1 is snapped to the boundary.

My shape editor is based loosely on jmcilhinney's 'manipulating GDI+ drawings' codebank entry, and a central idea in that post is to only draw what is necessary. In other words, when I move a shape I could re-draw the entire canvas, but it is much faster and smoother to only re-draw the old and the new location of the shape. Since nothing else has changed, I don't need to draw the rest. So, I simply invalidate the old bounds and the new bounds, and even with a lot of shapes all moving simultaneously it still feels very smooth.