The problem with this idea is that guides would no longer work as shape boundaries (which I use quite a bit for cropping and exporting). So you'd solve one problem while creating another.
This is a good point!
That's not a problem at all, because even 1px guides don't act as shape boundaries — they're ambiguous and treated completely differently in many apps. Of the 5 apps I tested, I noted 3 different behaviours.
Here's the results: http://i.imgur.com/fCM0Fep.png
So while you may think it's nice for guides to line up from a single pixel point of view with the edge of a shape, that's not possible either way. Far better to know for sure where a guide is than have it appear (maybe, possibly) right, but actually not be right.
Hey you're right!
Why hasn’t this been discussed much before? I bet others have noticed it, but I’ve never seen it mentioned. For the record, Skala will use 2px wide guides.
Also, the rules design apps use for snapping position vary wildly — I tested 5 apps and got 3 different results. So even if someone’s thinking “hey, it’s ok, I know everything snaps top right”, they’d better be paying attention if they change tools.
Edit Room's smart guides are 2 px wide.
Snapping rules are surprisingly complex to implement - there's a wide variety of possibilities and expectations for varying operations that might result in an element being snapped.
Great to hear you guys did it that way. I’d be so happy if others followed suit.
Ah, I used to have a rule of thumb, always snap things to the nearest whole pixel on the right, but 2px is much better!