Select a clip.
Add a Mask:from File filter.
Point to a file.
Works fine. But…
… decide to select “Invert”, or change “softness” or any other change…
… clicking anywhere in the Mask from File filter pane brings down the dropdown list, obscuring the options you want to change.
It only does this the first time.
Workaround is to click the pane twice anywhere before changing parameters but IMO this is a bug.