iris: a scroll area whose content grew asks to be drawn again

Iris's phone: typing newlines into the composer with the keyboard up
dropped the caret flush against the bar's bottom edge, eating the 12dp
padding, and closing the keyboard fixed it.

`Scroll::draw` offers its child last frame's content length on purpose,
so an ordinary scroll tick is an O(1) move rather than a redraw. The
comment claimed the lag self-corrects on the next frame; nothing asked
for that frame. A keystroke dirties the field, that frame draws it in a
box one line short of its text, and the tree is clean afterwards -- so
the stale placement is the last one drawn. The composer's text is
centred in its box, so one line short hung half a line past each end and
put the caret's line box a whole padding low. Closing the keyboard
rewrote the bar's inset, dirtied it, and forced the missing redraw.

`Scroll::draw` now calls `Painter::draw_again` when what it measured
differs from what it offered, and a frame that leaves anything dirty asks
for another frame on both backends -- `draw_again` sets its mark during
the update, after the input path's own check has run, so nothing asked
before this (which applied to `List::clamp_to_content` too).

Verified at layer 1 (the new test fails on the old code with the caret
exactly on the bar's edge) and on the emulator: the caret's bottom moved
from 1535 -- the bar's own bottom edge -- to 1509, 26px inside a 31px
padding, the remainder being parley's line box overhanging its line
height. `phone.rs` grew `--typed TEXT`, which enters text over frames
rather than preloading it; only that reproduces this.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/DECISIONS.md

@@ -5,6 +5,21 @@ they can be judged and reversed later. Detail lives in RUST.md (and IRIS.md
 for iris API changes); this file is only the summary. Newest first. Items
 marked **DEFERRED** are ones the agent chose not to decide alone.
 
+## 2026-09-08 (later: a scroll area that grew redraws once)
+
+From Iris's phone report about the composer's padding while typing
+newlines. IRIS.md's entry has the account.
+
+- **A `Scroll` whose measured content length differs from the length it
+  offered its child asks for one more draw** (`Painter::draw_again`),
+  rather than the stale box being the last one drawn. Pays one extra
+  draw of the scroll's subtree when the content's length changes --
+  including its first frame, where the offered length is a placeholder --
+  and nothing on an ordinary scroll tick.
+- **A frame that leaves any widget dirty now requests another frame** on
+  both backends. Previously only an animation did, so any use of
+  `draw_again` depended on some later input to deliver its correction.
+
 ## 2026-09-08 (every crate to its latest version, wgpu 28 -> 30)
 
 At Iris's request. RUST.md's "Every crate to its latest version" box has