fix: chat scroll fix

toju-app/src/app/domains/chat/domain/rules/auto-scroll.rules.spec.ts

@@ -1,4 +1,4 @@
-import { resolveAutoScrollBehavior } from './auto-scroll.rules';
+import { isStuckToBottom, resolveAutoScrollBehavior } from './auto-scroll.rules';
 
 describe('resolveAutoScrollBehavior', () => {
   const base = {
@@ -48,3 +48,28 @@ describe('resolveAutoScrollBehavior', () => {
     ).toBe('smooth');
   });
 });
+
+describe('isStuckToBottom', () => {
+  it('is stuck when exactly at the bottom', () => {
+    expect(isStuckToBottom(0)).toBe(true);
+  });
+
+  it('is stuck within the default sticky threshold', () => {
+    expect(isStuckToBottom(300)).toBe(true);
+    expect(isStuckToBottom(299)).toBe(true);
+  });
+
+  it('is not stuck once past the default threshold', () => {
+    expect(isStuckToBottom(301)).toBe(false);
+    expect(isStuckToBottom(2000)).toBe(false);
+  });
+
+  it('honours a custom threshold', () => {
+    expect(isStuckToBottom(80, 100)).toBe(true);
+    expect(isStuckToBottom(150, 100)).toBe(false);
+  });
+
+  it('treats negative overscroll distances as stuck', () => {
+    expect(isStuckToBottom(-20)).toBe(true);
+  });
+});

toju-app/src/app/domains/chat/domain/rules/auto-scroll.rules.ts

@@ -42,3 +42,21 @@ export function resolveAutoScrollBehavior(input: AutoScrollDecisionInput): AutoS
 
   return input.withinInitialGrace ? 'instant' : 'smooth';
 }
+
+/** Default pixel distance under which the list is considered stuck to bottom. */
+export const STICKY_BOTTOM_THRESHOLD = 300;
+
+/**
+ * Whether the scroll position is close enough to the bottom that the list
+ * should keep auto-pinning to the latest message. Negative distances (rubber
+ * band / overscroll) count as stuck.
+ *
+ * This is the predicate the message list uses to decide whether a content
+ * height change (late image/embed/plugin render) should re-pin to bottom.
+ */
+export function isStuckToBottom(
+  distanceFromBottom: number,
+  threshold: number = STICKY_BOTTOM_THRESHOLD
+): boolean {
+  return distanceFromBottom <= threshold;
+}