fix inner scroll behavior

src/utils/nested-scroll.ts

@@ -0,0 +1,72 @@
+/**
+ * Nested scroll sections favor the innermost one under the cursor.
+ *
+ * opentui bubbles a wheel event up the renderable tree, so every ancestor
+ * `ScrollBoxRenderable` that has room to move scrolls — nested sections (e.g.
+ * the episode-description scrollbox inside a page's list pane) scroll in
+ * lockstep. This patches the scrollbox's wheel handler so the innermost
+ * scrollbox under the cursor wins instead:
+ *
+ *   • The first scrollbox that can move in the wheel's direction scrolls and
+ *     stops propagation, so its ancestors don't also scroll.
+ *   • When it is already at its boundary it lets the next outer scrollbox
+ *     take over (wheel chaining), matching typical nested-scroll UX.
+ */
+
+import { ScrollBoxRenderable } from "@opentui/core";
+import type { MouseEvent } from "@opentui/core";
+
+type ScrollDir = "up" | "down" | "left" | "right";
+
+// The scrollbox's own wheel handler (scrolls, then bubbles to its parent).
+const original = ScrollBoxRenderable.prototype.onMouseEvent;
+
+let installed = false;
+
+/** True when `sb` has room to move in `dir` from its current position. */
+function canScroll(sb: ScrollBoxRenderable, dir: ScrollDir): boolean {
+	const maxTop = Math.max(0, sb.scrollHeight - sb.viewport.height);
+	const maxLeft = Math.max(0, sb.scrollWidth - sb.viewport.width);
+	switch (dir) {
+		case "up":
+			return sb.scrollTop > 0;
+		case "down":
+			return sb.scrollTop < maxTop;
+		case "left":
+			return sb.scrollLeft > 0;
+		case "right":
+			return sb.scrollLeft < maxLeft;
+	}
+}
+
+const handleWheel = function (
+	this: ScrollBoxRenderable,
+	event: MouseEvent,
+): void {
+	if (event.type !== "scroll" || !event.scroll?.direction) {
+		original.call(this, event);
+		return;
+	}
+
+	const dir = event.scroll.direction;
+	const effective: ScrollDir = event.modifiers.shift
+		? (dir === "up" ? "left" : dir === "down" ? "right" : dir === "right" ? "down" : "up")
+		: dir;
+
+	const moves = canScroll(this, effective);
+	original.call(this, event);
+	// Only claim the wheel when this box actually moved; otherwise let the
+	// next outer scrollbox (also under the cursor) take over.
+	if (moves) event.stopPropagation();
+};
+
+export function installNestedScrollBehavior(): void {
+	if (installed || typeof original !== "function") return;
+	installed = true;
+	// `onMouseEvent` is a well-known protected method; the cast only bypasses
+	// TypeScript's protected-access check and trusts the shipped class shape.
+	const scrollboxProto = ScrollBoxRenderable.prototype as unknown as {
+		onMouseEvent: typeof handleWheel;
+	};
+	scrollboxProto.onMouseEvent = handleWheel;
+}