initial commit

fluxer_app/src/lib/scroll/scrollPosition.ts

@@ -0,0 +1,62 @@
+/*
+ * Copyright (C) 2026 Fluxer Contributors
+ *
+ * This file is part of Fluxer.
+ *
+ * Fluxer is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Fluxer is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with Fluxer. If not, see <https://www.gnu.org/licenses/>.
+ */
+
+export interface ScrollMetrics {
+	scrollTop: number;
+	scrollHeight: number;
+	offsetHeight: number;
+}
+
+export interface ScrollPinOptions {
+	tolerance?: number;
+	stickyTolerance?: number;
+	hasMoreAfter?: boolean;
+	wasPinned?: boolean;
+	allowPinWhenHasMoreAfter?: boolean;
+}
+
+export interface ScrollPinResult {
+	distanceFromBottom: number;
+	isAtBottom: boolean;
+	isPinned: boolean;
+}
+
+const DEFAULT_TOLERANCE = 8;
+const DEFAULT_STICKY_TOLERANCE = 64;
+
+export function evaluateScrollPinning(metrics: ScrollMetrics, options: ScrollPinOptions = {}): ScrollPinResult {
+	const tolerance = options.tolerance ?? DEFAULT_TOLERANCE;
+	const stickyTolerance = options.stickyTolerance ?? DEFAULT_STICKY_TOLERANCE;
+	const hasMoreAfter = options.hasMoreAfter ?? false;
+	const allowPinWhenHasMoreAfter = options.allowPinWhenHasMoreAfter ?? true;
+	const wasPinned = options.wasPinned ?? false;
+
+	const distanceFromBottom = Math.max(metrics.scrollHeight - metrics.offsetHeight - metrics.scrollTop, 0);
+	const isWithinTolerance = distanceFromBottom <= tolerance;
+	const isWithinStickyRange = distanceFromBottom <= stickyTolerance;
+
+	const shouldPin =
+		(isWithinTolerance || (wasPinned && isWithinStickyRange)) && (allowPinWhenHasMoreAfter || !hasMoreAfter);
+
+	return {
+		distanceFromBottom,
+		isAtBottom: isWithinTolerance,
+		isPinned: shouldPin,
+	};
+}