When resizing canvas guide lines position should follow original canvas according to anchor selection. At present time they stay at their absolute positions, which moves them in different places comparing to original place. I think that it is hard to find practical scenario when present behaviour is useful. I my opinion guide lines should follow original content. So if I had vertical guide line at 100px and canvas was 200px x 200 px. Resize it to 400px x 400 px with anchor set to center up, should move guide line to 200 px.