Open pdf manual
Press “Search” TAB
Type in... strangely enough “lock height”
Press “list topics”
Double click on… “Configuring the display of tracks in the track view”
Read last to lines on page…
"When you lock the height of a track, its height does not change when you use a Zoom or Fit command. When a track’s height is locked, you can still drag the track strip’s lower border to adjust the track’s height. After you drag the border, the altered track height becomes the track’s locked height."