I saw a couple suggestions to add a zoom lock (hey, palm rejection isn't perfect) and wanted to add an extra suggestion (for touch devices such as iPad). Add a button for locking zoom/horizontal scrolling and change the single-finger double tap from smart zoom to another tool, such as eraser, when the zoom is locked.

So with the zoom unlocked, double tap would work as smart zoom, just as it currently does. With zoom locked, the double tap could switch tools instead of having to click on the toolbar every time. Could be a massive time saver in the long run and since the double-finger double tap code is already written this might not be too difficult to implement (wishful thinking I know)!