Bold makes text wider, and a toolbar centred on the selection slides with it. The obvious fix made it worse. The right one came from asking what never moves.
Select a phrase, press Bold, then reach for Italic. The toolbar opens centred over the selection. Bold widens the words, the toolbar re-centres, and Italic is no longer where your pointer is heading. Fitts's law cares about where a target is when you arrive, not when you set off, so a target that moves mid-reach is a miss waiting to happen.
Press Bold a few times and watch where the Italic button goes.
The first instinct is to freeze the toolbar where it opened for as long as the selection lasts. It stops moving. It also stops telling the truth: after a couple of marks it hangs off one side of a selection that has grown past it, and spatial consistency between a control and its object is gone.
So freezing trades one broken promise for another. The toolbar keeps still, but it no longer belongs to the words.
Bold changes how wide a selection is, never where it starts. Anchor the toolbar to that stable edge, re-measure after every change, and both promises hold: nothing moves under the pointer, and the toolbar still sits on the text it acts on.
A toolbar pinned to the start edge reads slightly off-centre on a long selection. For a control you're about to press twice in a row, I'll take steady over symmetrical every time.
Draft. The reasoning above is scaffolding; rewrite it in your own words before publishing.