A hydrated page can start mid-turn: the first row is the tool-only
assistant, the second its tool result, and the answer arrives third with
no active bubble to append to. The bubble that fold creates stands for
all three backend rows, but only the two pending rows were counted
(serverRowSpan: 2), so the release rewound the older-page offset short
of what it gave up and 'Show earlier' skipped rows the reader could no
longer reach.
Count the current answer row too, and pin it with a regression whose
page starts on the tool-only assistant/result pair.
Found in review of 72a5aa3fd2 by @ehz0ah.
6668ac14a0