Select several tabs (such that the remaining tabs will not fully fill the strip) and drag them out of the current window.  The new tab button snaps to its new ideal bounds, while the remaining tabs animate; there's a period where they're overlapping.

This doesn't happen when inserting tabs into the strip: the NTB animates its ideal bounds in that case.

->rameier since this is a bit like some other bugs he dealt with already.