Hi there Brian,
It sounds like iMovie is recognizing that you have the cursor in the correct place to resize this pane but it doesn't actually move when you click, hold, and drag it. I would recommend testing this in a new standard user on the computer:
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Choose Apple menu > System Preferences, then click Users & Groups.
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Click the lock icon
to unlock it, then enter an administrator name and password.
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Click the Add button
below the list of users.
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Click the New Account pop-up menu, then choose a type of user.
Standard: Standard users are set up by an administrator. A standard user can install apps and change settings for his or her own use. Standard users can’t add other users or change other users’ settings.
Let me know how that goes, and also I could use the version number of iMovie to further assist you here.
CLOS