"If you are on the selected Tab when it becomes disabled, you should remain on this Tab."
The only question - is it still closable or not?
I think, it should become unclosable.
Because, if you switch to any other tab, disabled one become unclosable.And I think, possibility to close tab should depend on that, which tab is currently selected and which is disabled.
But now it stays closable. If you switch out the tab to another tab, it become unclosable. It doesn't look right.