Editing : Edit Modes and Tools : Zooming with a Scroll Wheel

Zooming with a Scroll Wheel
If you have a mouse with a scroll wheel, you can use the scroll wheel to zoom in and out of tracks, both vertically and horizontally.
To continuously zoom horizontally using a scroll wheel:
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Hold Alt (Windows) or Option (Mac) while turning the mouse scroll wheel.
To continuously zoom audio (vertical zoom) in the Edit window:
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Hold Alt+Shift (Windows) or Option+Shift (Mac) while turning the mouse scroll wheel.
To continuously zoom MIDI (vertical zoom) in the Edit window:
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Hold Alt+Start (Windows) or Option+Control (Mac) while turning the mouse scroll wheel.
To scroll the contents of the Edit window or Mix window horizontally:
1 Focus the window you want to scroll by clicking in it or bringing it forward.
2 Hold the Shift key while turning the mouse scroll wheel.