A tooltip keeps the text readable
Side panels force a choice: read the draft or read the findings. A tooltip avoids it. The text stays at full measure with one dotted underline per problem, and the explanation appears only where you look, then leaves.
Floating UI does the part that usually breaks: collision detection. Near the top of the viewport the tooltip flips below the sentence, and near an edge it shifts inside, so a diagnostic is never cut off or hidden behind the line it describes.