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If you spectate a game you can use the M3 or the mousewheel button to smoothly and precisely adjust your view.
However if you go into a match and play and do the same thing, the M3 button scrolling will demonstrate and huge dead-zone in which using the M3 scrolling is almost the same as the putting-cursor-on-edge-of-screen scrolling.
Maybe it's not a bug, but if it wasn't it would be redundant to have two ways to scroll the map in which one way is absolutely more difficult.
The kind of smooth scrolling you get from M3 in spectate mode would be awesome to have in real games. If there is enough support maybe this problem will get noticed :P