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