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');
+  });
+});

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

@@ -0,0 +1,44 @@
+/** Behaviour the message list should apply when new messages arrive. */
+export type AutoScrollBehavior = 'smooth' | 'instant' | 'none';
+
+export interface AutoScrollDecisionInput {
+  /** Whether the message count grew since the last render. */
+  readonly newMessages: boolean;
+  /** Forced jump for the local user's own just-sent message. */
+  readonly forceLocalSend: boolean;
+  /** Pixels from the bottom of the scroll container. */
+  readonly distanceFromBottom: number;
+  /**
+   * True while the conversation is still settling after a channel/server
+   * switch (initial scroll + async embed/image height changes). During this
+   * window we jump instantly so we never animate to the wrong position.
+   */
+  readonly withinInitialGrace: boolean;
+  /** Distance threshold under which the list is considered stuck to bottom. */
+  readonly stickyThreshold?: number;
+}
+
+/**
+ * Decides how the message list should react when the message count changes.
+ *
+ * - No new messages -> `none`.
+ * - Local user's own send -> `instant` (never animate your own message).
+ * - Near bottom during the post-switch settle window -> `instant` (so loading
+ *   a channel always lands at the bottom without a mid-chat smooth animation).
+ * - Near bottom afterwards -> `smooth` (live messages animate into view).
+ * - Far from bottom -> `none` (show the new-messages indicator instead).
+ */
+export function resolveAutoScrollBehavior(input: AutoScrollDecisionInput): AutoScrollBehavior {
+  if (!input.newMessages)
+    return 'none';
+
+  if (input.forceLocalSend)
+    return 'instant';
+
+  const threshold = input.stickyThreshold ?? 300;
+
+  if (input.distanceFromBottom > threshold)
+    return 'none';
+
+  return input.withinInitialGrace ? 'instant' : 'smooth';
+}