Seekbar header is not recomputed after fixing identities

[talmo]

Bug description

Updating track identities by assigning instances to tracks does not update the seekbar header.

Expected behaviour

Seekbar header features should be recomputed after fixing identities.

This can probably occur in a background thread so the GUI isn't slowed down.

[roomrys]

Hi @talmo,

Could you provide an example to recreate the problem. I "recently" tried recreate, but did not notice the error you mention.

Thanks, Liezl