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Opening the keyboard in a busy channel janked: the composer's bottom inset changes on every keyboard animation frame, and each change rebuilt the message list, which rebuilt every visible message row. On an iPhone 12 mini that came to 8 frames over 16 ms per keyboard open, with 290 to 360 ms spent in build. The same cascade runs on every page rebuild, such as the dozen or so while a large channel settles after returning to the app. Flutter skips a rebuild only when it gets the identical widget object back, so the message list now keeps each row it built and returns it while nothing the row is built from has changed. Shared inputs (the message list, membership, channel names, send animations, the focus callback, today's date and so on) clear the cache when they change. Each row also compares its neighbors, thread summary, typing and working state, and send-animation and handoff flags. Rows still rebuild through their own provider watches. The composer's focus callback is now stable so the page's rebuilds don't clear the cache. ### Validation - Added `a growing composer reuses the message rows on screen`: it types a four-line message and checks that the visible rows are the same widget objects. It fails without the cache. - `flutter analyze` is clean. The full mobile suite passes except the 10 push and notification-toggle tests that fail identically on unchanged main on this machine. - iPhone 12 mini, Profile build, a 2,000-member channel, one matched run each before and after (the after column is an earlier revision of this change with the same mechanism): | | Before | After | | --- | --- | --- | | Keyboard open, build time | 359 ms, 290 ms | 64 ms, 43 ms | | Keyboard open, frames over 16 ms | 8, 4 | 1, 0 | | Return to the app, build time | 953 ms | 881 ms | | Return to the app, frames over 16 ms | 21 | 18 | Most of what remains on return comes from rows' own watches (for example the agent display-name map is a new object on every agent directory reload), which this change doesn't address. - Automated app run on the same phone with this exact branch: Home, opening the large channel, opening the keyboard twice (58 and 69 ms of build, one frame over 16 ms each), scrolling back through several days of history, and returning to the app after 12 s in the background. Day dividers, reply counts, reactions and system rows rendered correctly throughout. Typing is covered by the widget test rather than the device run, to avoid drafting in a real channel. - Simulator QA of this branch (Debug build, a private test channel): typing a four-line message, sending, scrolling to the top and back, a reaction, an edit, a thread reply, a delete, sending again, and backgrounding and returning. Every row rendered and updated correctly, with no replayed send animations. The jump-to-latest button didn't respond to synthetic simulator taps before or after resume; it's a native iOS view this change doesn't touch. https://github.com/user-attachments/assets/04909261-81fe-4514-98aa-8d7fdfdcace5 🤖 Generated with [Claude Code](https://claude.com/claude-code) --------- Signed-off-by: Tom Brow <tomb@squareup.com> Co-authored-by: Tom Brow <tomb@squareup.com> Co-authored-by: studio <dc2fc481343672df31a689840c09299972e704dac592e0da6f18486646a7ee3e@buzz.block.builderlab.xyz>