Very open to suggestions, but this was the most straight-forward way to hook into the behavior at the moment.

The specific problem I found was that modifying the `absolute_right` and `absolute_top` for the `TextBlock` to get the proper next positioning actually seemed to result in it moving when things rendered. It also seemed to fight with what the dimensioning delegates.

Very possible I'm missing a better abstraction here, though, as I agree it's a bit invasive to have the `Slot` handing over that fully to the individual element here.