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

2026-06-02 20:25:06 +02:00
parent f73abbd860
commit 43a99a8d6d
2 changed files with 100 additions and 0 deletions
@@ -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' };
+}