fix(web): the fade follows measured overflow, so text that fits is not dimmed forever

[gbasin]

Found while browser-verifying #545 after it landed.

The bug

#545 made the toggle depend on measured overflow instead of the length heuristic — good — but left the fade on the collapsed state alone:

const clamped    = canClamp && !expanded;   // fade rode on this
const showToggle = canClamp && overflows;   // toggle rode on this

clamped applies collapsedClassName, and for MessageText that string carried the fade mask. So a message long enough to trip COLLAPSE_LINE_THRESHOLD (16 lines) or COLLAPSE_CHAR_THRESHOLD (1800 chars) but short enough to fit inside max-h-80 renders with the bottom of its box faded to transparent and no toggle to clear it. The text is simply dimmed, permanently.

Before #545 the toggle was gated on the heuristic too, so that message faded but could be expanded — the fade was escapable. #545's note that this case "no longer offers a toggle that does nothing" understates it: the toggle was doing something visible. It removed the fade.

Easy to hit with markdown that renders more compactly than its source — blank-line-padded text is the obvious one.

The fix

The constraint has to stay applied while collapsed, because overflow is measured through it: drop max-h-80 and scrollHeight === clientHeight, so nothing ever reports overflowing and the block never clamps at all. Gating collapsedClassName on overflows would be circular.

So the two concerns split:

• collapsedClassName — the size constraint, applied whenever collapsed (needed to measure)
• overflowingClassName — the hint that there is more to see, applied only when it actually overflows

MessageRow's clamp is unaffected: line-clamp-3 is a no-op when the content fits, so it never needed the split.

Verification

Real browser (Chromium, built + preview), reading computed style — jsdom has no layout and cannot see a mask:

case	overflows	Show more	mask-image before	after
trips heuristic, fits (76px in a 320px box)	false	0	linear-gradient(rgb(0,0,0) 70%, rgba(0,0,0,0))	none
genuinely overflows (933px in 320px)	true	1	present	present ✓
below threshold	—	0	not clamped	not clamped ✓

Also confirmed #545's own win is intact — "Show more" still grows the reply (111px → 725px → 111px), nowhere near the #538 shrink (360 → 87).

• pnpm lint ✓ · pnpm -r typecheck ✓ · 761 web unit tests ✓
• e2e cluster-steer-interleave + session-pane-ux + spine-aside: 7 passed
• Unit tests added for both directions (the existing suite stubs scrollHeight/clientHeight + ResizeObserver, so the class-level behaviour is testable even though the visual is not)