Hi,
after searching a bit for a circular progress indicator component that’s constantly spinning I found out by “accident” that the ProgressBar component does this when the progress value passed is -1. However, I didn’t find any mention of that feature in the documentation when I was searching for something like that. Maybe it would be useful for others to add a note about this feature in the doc - for me, it would have saved half an hour of work 
ProgressBar has a range of 0 - 1.0 for its progress value so I guess technically anything outside of that is undefined behaviour. However, the JUCE LookAndFeels are nice enough to draw the spinny animation for these values so I’ve added that to the documentation.
Are you sure that this is no defined behaviour? Because in the Projucer source code, the same trick is done to generate the spinning indicator that is showed when clicking the “Save project and open in IDE” button - take a look here: https://github.com/WeAreROLI/JUCE/blob/master/extras/Projucer/Source/Project%20Saving/jucer_ProjectSaver.h
Edit: read your post twice and realized that you mentioned that look & feel is responsible to handle it this way. Sorry, was reading the post on my phone. Thank you for adding the comment 
