Custom Solid Generator doesn't always fill the screen.

I'm been making Instagram videos recently which are 640x640.
I had been using a 720p sequence and created a 640x640 inverted box mask to help with framing, then cropping the master files in handbrake. But custom canvas sizes in 10.1 is a great opportunity to do it the easy way.

However when I make a white solid fill on the timeline (using the built in generator) it doesn't quite fill the whole frame but leaves a small uneven border. So it has to be scaled up by a few percent.
I've tried adjusting the spacial controls just in case, to no avail.

Some other non standard sizes I've been using also exhibit the effect, but 1080 seems OK.
It's not a deal breaker and I've submitted a report to Apple, but at some time I know it could cause issues when matching crops matting with a solid or doing some effect or other.

Can anyone else confirm this issue?
It's a bug not a feature I guess? :silly:

It is a difference in a non-standard aspect ratio, I believe is what you're seeing, not a bug.

Hi Ben, honestly I don't think the non-standard frame size is the problem here.
The other 5 generators in the 'Solids' section fill a 640x640 timeline perfectly.

I've tried fiddling with an additional snapshot size in a copy of the Generator's motion file but I'm not getting anywhere with it.