- index.html: btnSend hidden by default (display:none), icon-only (upward
arrow SVG, no text label), title attribute for accessibility
- style.css: new send-btn design — 34px circle, blue fill (#7cb9ff),
subtle glow box-shadow, scale() hover/active for tactile feel,
.send-btn.visible with @keyframes send-pop-in (scale+opacity spring
using cubic-bezier(.34,1.56,.64,1) for a satisfying pop). Mobile
override updated to preserve circle dimensions.
- ui.js: updateSendBtn() — shows button with pop-in animation when
textarea has content OR files are attached and agent is not busy;
hides instantly when content is cleared. Hooked into setBusy() and
renderTray() so button state tracks all content sources correctly.
- boot.js: input event listener calls updateSendBtn() on every keystroke.
- messages.js: autoResize() calls updateSendBtn() so button disappears
immediately after send clears the textarea.
- tests/test_sprint21.py: 33 tests covering HTML structure, CSS design
(circle shape, colors, animations, keyframes), JS logic (updateSendBtn,
setBusy, renderTray, autoResize integration), and regressions
(363 total, all pass).
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