fix: Improve autoscroll to bottom

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

@@ -0,0 +1,50 @@
+import { resolveAutoScrollBehavior } from './auto-scroll.rules';
+
+describe('resolveAutoScrollBehavior', () => {
+  const base = {
+    newMessages: true,
+    forceLocalSend: false,
+    distanceFromBottom: 0,
+    withinInitialGrace: false
+  };
+
+  it('does nothing when no new messages arrived', () => {
+    expect(resolveAutoScrollBehavior({ ...base, newMessages: false })).toBe('none');
+  });
+
+  it('jumps instantly for the local user own send regardless of grace', () => {
+    expect(resolveAutoScrollBehavior({ ...base, forceLocalSend: true })).toBe('instant');
+    expect(
+      resolveAutoScrollBehavior({ ...base, forceLocalSend: true, withinInitialGrace: true })
+    ).toBe('instant');
+  });
+
+  it('jumps instantly when near bottom while settling after a channel switch', () => {
+    expect(
+      resolveAutoScrollBehavior({ ...base, distanceFromBottom: 40, withinInitialGrace: true })
+    ).toBe('instant');
+  });
+
+  it('animates smoothly for live messages once settled and near bottom', () => {
+    expect(
+      resolveAutoScrollBehavior({ ...base, distanceFromBottom: 40, withinInitialGrace: false })
+    ).toBe('smooth');
+  });
+
+  it('shows the indicator (no scroll) when far from the bottom', () => {
+    expect(resolveAutoScrollBehavior({ ...base, distanceFromBottom: 800 })).toBe('none');
+    expect(
+      resolveAutoScrollBehavior({ ...base, distanceFromBottom: 800, withinInitialGrace: true })
+    ).toBe('none');
+  });
+
+  it('honours a custom sticky threshold', () => {
+    expect(
+      resolveAutoScrollBehavior({ ...base, distanceFromBottom: 150, stickyThreshold: 100 })
+    ).toBe('none');
+
+    expect(
+      resolveAutoScrollBehavior({ ...base, distanceFromBottom: 80, stickyThreshold: 100 })
+    ).toBe('smooth');
+  });
+});