maximising the dash causes the blurred background to "jump" downwards

What happens:

Opening the dash maximised, or opening it minimised and then maximising it, causes the blurred background to visibly jump downwards by a small distance.

What should happen:

Opening the dash maximised should NOT cause the blurred background to offset at all.

To reproduce this, open the dash and then maximise it. You should see the windows and applications you were using offset downwards slightly.

Looking at it closely, the "jump" leaves behind a 'ghost'. To reproduce this, open up Firefox and open 4 or 5 tabs on websites with colourful icons. Facebook, omgubuntu.co.uk and Wikipedia will do. Open the dash and then repeatedly minimise and maximise it. When the blurred background jumps downwards, you will see faint coloured blobs hovering over the blurred tab icons.

(Another element is that possibly the faint icon reflections are also being wrongly offset, but it's hard to judge this because the reflections may be realigning to the changed dash size anyway. They're also very hard to see at the best of times.)

I'm attaching a screencast showing this behaviour, but unfortunately Unity isn't behaving well and there's lots of flickering. Hopefully someone else without this problem might record footage of the same behaviour and upload.

I'm using an Nvidia card with the binary blob and running 12.04 with unity 5.10.