#4856 (mobile transcript completely unscrollable) survived multiple fixes
because regressions were guarded only by grepping one exact CSS string. This
adds an INVARIANT guard so the whole class can't recur: .messages-inner (base +
every media query) must never become a Y-axis scroll container — neither via an
explicit overflow-y scroll value nor via overflow-x:hidden/scroll/auto (which
the CSS spec coerces overflow-y->auto, collapsing min-height:auto to 0 inside
the flex column). overflow-x:clip is the only allowed horizontal clip. Also
asserts .messages stays the real scroller (overflow-y:auto + min-height:0).
Proven non-vacuous: reintroducing overflow-x:hidden on the mobile rule fails it.
For a true runtime check at a real viewport, the private workspace has
scripts/layout_scroll_probe.py.
nesquena-hermes