fix(assistant): #91 maintien de l'ancre en haut pendant le streaming, libération au scroll manuel

docs/features/ai-assistant-conversation-ux.md

@@ -26,9 +26,11 @@
      l'ouverture d'un contexte, au rechargement d'une session et à la reprise après
      confirmation agent : cible = dernier `.bookslm-msg.user`, `scrollIntoView`
      fluide `block: 'start'`, repli calcul `scrollTop += delta` si indisponible.
-  - [x] **A2.** Re-rendus de streaming : la position de scroll est **préservée**
-     (plus de `scrollTop = scrollHeight`) — la réponse s'allonge sous la question
-     sans jamais déplacer la vue.
+  - [x] **A2.** Re-rendus de streaming : tant que la réponse arrive (`_isLoading`) et
+     que l'utilisateur n'a pas touché à la molette (flag `_pinnedTurn`, libéré sur
+     `wheel`/`touchmove`), chaque frame re-ancre instantanément la question en haut —
+     la réponse s'allonge sous la question sans jamais déplacer la vue. Hors
+     streaming, la position de scroll est préservée (plus de `scrollTop = scrollHeight`).
   - [x] **A3.** Marge supérieure : `scroll-margin-top: 14px` sur `.bookslm-msg`
      pour que le post ne colle pas au bord.
   - [x] **A4.** Seule la zone du fil défile : `.bookslm-messages { flex: 1; overflow-y: auto }`

frontend/js/bookslm.js

@@ -122,6 +122,9 @@ class BooksLM {
     this._menuSeq = 0;
     // Timer that hides the "done" activity indicator after a short delay.
     this._activityHideTimer = null;
+    // True while the latest question should stay pinned to the top of the
+    // thread (cleared as soon as the user scrolls manually).
+    this._pinnedTurn = false;
     // Cache of every indexed path per vault, backing instant `@` filtering.
     this._pathCache = null;
     this._pathCacheVault = null;
@@ -887,6 +890,14 @@ class BooksLM {
       this._activatePath(path, kind);
     });
 
+    // A manual scroll releases the top-pinning of the latest question.
+    const messagesEl = panel.querySelector('.bookslm-messages');
+    if (messagesEl) {
+      const unpin = () => { this._pinnedTurn = false; };
+      messagesEl.addEventListener('wheel', unpin, { passive: true });
+      messagesEl.addEventListener('touchmove', unpin, { passive: true });
+    }
+
     // Close the session / command menus when clicking elsewhere in the panel.
     panel.addEventListener('click', (e) => {
       if (!e.target.closest('.bookslm-history-menu') && !e.target.closest('.bookslm-btn-history')) {
@@ -1793,16 +1804,20 @@ class BooksLM {
     }
 
     if (opts.anchor) this._anchorLatestUser();
+    else if (this._isLoading && this._pinnedTurn) this._anchorLatestUser({ instant: true });
     else container.scrollTop = prevTop;
   }
 
   /**
    * Scroll the messages window so the most recent user post sits at the very
    * top of the viewport (its `scroll-margin-top` keeps a breathing gap),
-   * reproducing Notion-AI's conversation anchoring.
+   * reproducing Notion-AI's conversation anchoring. The pin stays active
+   * while the answer streams, so the question never leaves the top — until
+   * the user scrolls manually (see the wheel/touch listeners in `_render()`).
    */
-  _anchorLatestUser() {
+  _anchorLatestUser(opts = {}) {
     if (!this._panel) return;
+    this._pinnedTurn = true;
     const container = this._panel.querySelector('.bookslm-messages');
     if (!container) return;
     const users = container.querySelectorAll('.bookslm-msg.user');
@@ -1810,9 +1825,10 @@ class BooksLM {
     // Fallback (admin notes with no user post): anchor the newest message.
     const el = users[users.length - 1] || all[all.length - 1] || container.firstElementChild;
     if (!el) return;
+    const behavior = opts.instant ? 'auto' : 'smooth';
     const apply = () => {
       try {
-        el.scrollIntoView({ behavior: 'smooth', block: 'start' });
+        el.scrollIntoView({ behavior, block: 'start' });
       } catch {
         const cRect = container.getBoundingClientRect();
         const eRect = el.getBoundingClientRect();

tests/frontend/ai.test.mjs

@@ -304,7 +304,25 @@ async function main() {
       panel.remove();
     });
 
-    await test("empty content renders no action bar", () => {
+    await test("manual wheel scroll releases the top-pinning", async () => {
+    const b = new BooksLM();
+    const panel = b._render();
+    b._panel = panel;
+    document.body.appendChild(panel);
+    b._messages = [
+      { role: "user", content: "premier" },
+      { role: "assistant", content: "réponse" },
+      { role: "user", content: "deuxième" },
+    ];
+    b._renderMessages({ anchor: true });
+    assert.equal(b._pinnedTurn, true, "anchor sets the pin");
+    const container = panel.querySelector(".bookslm-messages");
+    container.dispatchEvent(new w.WheelEvent("wheel", { deltaY: -30, bubbles: true }));
+    assert.equal(b._pinnedTurn, false, "wheel releases the pin");
+    panel.remove();
+  });
+
+  await test("empty content renders no action bar", () => {
       const b = new BooksLM();
       const panel = b._render();
       b._panel = panel;