I would like to suggest adding horizontal scroll in the layout editor using [Shift + Mouse Wheel]. This would make it much easier to move around the layout when editing large layouts or when zoomed in.
I believe this has been suggested before, but I don't recall the results of those suggestions.
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I don't see what's wrong with the current method of using the middle mouse button - it's much simpler than having to hold a key down as well as use the mouse.
If you want a key/mouse combo, then it's currently set to SPACE + mouse move.
I use Microsoft Visio a lot, in conjunction with C2, for design and diagramming. I find using the Shift + Mouse Wheel combo very useful, especially when I'm flipping back and forth between the two. I find using the space-bar combo to not be as easy.
I don't see what's wrong with adding a bit more functionality to the editor. Shift + Mouse Wheel is used in a lot of other applications for horizontal scrolling, including most web browsers.