Chat scroll smoke test fails on main since the tail reserve #1783

ChatTabViewSmokeTests.Scrolling_uneven_history_does_not_reverse_the_scrollbar(False, False) fails on main on every OS since 93ca913 (#1766), which also fails every open PR's `Build and test`.

At the scroll end, ChatTailPanel reserves blank space below the latest user turn, so no row covers the viewport's middle. The test looks up its "reading row" only at the exact middle, finds none for the first wheel steps, and records step N: reading row moved px or was recycled.