When you click a video clip in an Event (source video), iMovie automatically selects a preset range of video frames. This automatic range selection can be useful, for example, when you want to create video clips of precisely equal size to time them to correspond with the rhythm of a musical soundtrack.
By default, the length of the automatic selection is four seconds of video. In iMovie preferences, you can change this default setting so that clicking an Event clip selects a minimum of one second of video or a maximum of ten seconds of video. You can also set preferences so that clicking in an Event selects entire clips.
To set the automatic frame range selection in source video:
Choose iMovie > Preferences, click Browser, and then do one of the following:
To change the length of the selection, select “Clicking in Event Browser selects,” and then drag the slider to set how many seconds you want to select.
To set the automatic selection to entire clips, select “Clicking in Event Browser selects entire clip.”
To turn off automatic selection, select “Clicking in Event Browser deselects all.”