feat(desktop): pure scroll-decision logic for new message list

apps/desktop/src/lib/scrollController.test.ts

@@ -0,0 +1,57 @@
+import { describe, expect, it } from 'vitest';
+
+import { isNearBottom, isNearTop, resolveInitialAnchor } from './scrollController';
+
+const m = (scrollTop: number, scrollHeight: number, clientHeight: number) => ({
+  scrollTop,
+  scrollHeight,
+  clientHeight,
+});
+
+describe('isNearBottom', () => {
+  it('true exactly at the bottom', () => {
+    expect(isNearBottom(m(900, 1000, 100), 64)).toBe(true);
+  });
+  it('true within threshold', () => {
+    expect(isNearBottom(m(860, 1000, 100), 64)).toBe(true);
+  });
+  it('false beyond threshold', () => {
+    expect(isNearBottom(m(800, 1000, 100), 64)).toBe(false);
+  });
+});
+
+describe('isNearTop', () => {
+  it('true at top', () => {
+    expect(isNearTop(m(0, 1000, 100), 64)).toBe(true);
+  });
+  it('false past threshold', () => {
+    expect(isNearTop(m(200, 1000, 100), 64)).toBe(false);
+  });
+});
+
+describe('resolveInitialAnchor', () => {
+  it('anchors to last row at end by default (no saved position)', () => {
+    expect(resolveInitialAnchor(null, 50)).toEqual({ index: 49, align: 'end' });
+  });
+  it('anchors to bottom when saved position stuck to bottom', () => {
+    expect(resolveInitialAnchor({ topmostIndex: 10, stickToBottom: true }, 50)).toEqual({
+      index: 49,
+      align: 'end',
+    });
+  });
+  it('restores the saved row at the top when scrolled up', () => {
+    expect(resolveInitialAnchor({ topmostIndex: 12, stickToBottom: false }, 50)).toEqual({
+      index: 12,
+      align: 'start',
+    });
+  });
+  it('clamps a stale saved index to the current row count', () => {
+    expect(resolveInitialAnchor({ topmostIndex: 999, stickToBottom: false }, 50)).toEqual({
+      index: 49,
+      align: 'start',
+    });
+  });
+  it('handles an empty list', () => {
+    expect(resolveInitialAnchor(null, 0)).toEqual({ index: 0, align: 'end' });
+  });
+});

apps/desktop/src/lib/scrollController.ts

@@ -0,0 +1,43 @@
+// Pure, DOM-free scroll-decision logic for MessageList. Unit-tested so the
+// tricky math is verified without a browser (jsdom has no layout).
+
+export interface ScrollMetrics {
+  scrollTop: number;
+  scrollHeight: number;
+  clientHeight: number;
+}
+
+/** Distance from the bottom edge is within `threshold` px. */
+export function isNearBottom(m: ScrollMetrics, threshold: number): boolean {
+  return m.scrollHeight - (m.scrollTop + m.clientHeight) <= threshold;
+}
+
+/** Scroll offset is within `threshold` px of the top. */
+export function isNearTop(m: ScrollMetrics, threshold: number): boolean {
+  return m.scrollTop <= threshold;
+}
+
+export interface SavedPosition {
+  topmostIndex: number;
+  stickToBottom: boolean;
+}
+
+export interface Anchor {
+  index: number;
+  align: 'start' | 'end';
+}
+
+/**
+ * Where a freshly-opened chat should start.
+ * - default / "left at bottom" → last row, aligned to the viewport bottom.
+ * - "left scrolled up" → the saved top-most row, aligned to the viewport top
+ *   (clamped in case the cached row count shrank).
+ */
+export function resolveInitialAnchor(saved: SavedPosition | null, rowCount: number): Anchor {
+  if (rowCount <= 0) return { index: 0, align: 'end' };
+  if (saved && !saved.stickToBottom) {
+    const index = Math.max(0, Math.min(saved.topmostIndex, rowCount - 1));
+    return { index, align: 'start' };
+  }
+  return { index: rowCount - 1, align: 'end' };
+}