Most people know that using the waveform zoom buttons in the toolbar you can zoom waveforms vertically. Since the introduction of clip gain there has been the potential for confusion as there are now two possible reasons why waveform height has changed. Zooming is straightforward but its not at all obvious how to get back to where you started.
Reset Waveform Zoom - Without Changing Horizontal Zoom
To reset your waveforms back to the default level of zoom simply use CMD+Opt+Ctrl+[ on a mac or Ctrl+Alt+Start+[ on a PC. Alternatively you could double-click the Zoom tool or use Option/Alt+A to achieve almost the same result. While those do reset waveform zoom, double-clicking the Zoom tool or using Alt+A also zooms out horizontally to show your entire timeline.
Extra Tip - Waveform Zoom On A Single Track
In Pro Tools, normally waveform zoom is global - there is one setting which affects all waveforms in the session. However there is a way to zoom in to different amounts on different tracks.
Select the zoom tool, hold Control or Start on a PC and drag vertically, this works on an many tracks as you choose to use on on so you can, for example zoom in on some tracks, out on others and leave the rest at their default. If you set up different levels of waveform zoom and then change the global zoom the zoom offset between tracks persists until the maximum or minimum is reached and if you push past that the offsets will get removed.