src/components/messageStream/MessageStream.tsx

@@ -1,4 +1,4 @@
-import { useEffect, useMemo, useRef } from "react";
+import { useEffect, useMemo, useRef, useState } from "react";
 import { useLocation, useSearchParams } from "react-router-dom";
 import { postJSON } from "../../api";
 import { useI18n } from "../../i18n";
@@ -79,23 +79,72 @@ export function MessageStream({ scope }: MessageStreamProps) {
     ? hash.slice("#post-".length)
     : "";
   const targetPostId = queryTargetPostId || hashTargetPostId;
+  const [isAligningQueryTarget, setIsAligningQueryTarget] = useState(
+    Boolean(queryTargetPostId),
+  );
   const handledTargetRef = useRef<string>("");
   const targetScrollTimersRef = useRef<number[]>([]);
+  const targetScrollFrameRef = useRef<number | null>(null);
 
   const clearTargetScrollTimers = () => {
     for (const timer of targetScrollTimersRef.current) {
       window.clearTimeout(timer);
     }
     targetScrollTimersRef.current = [];
+    if (targetScrollFrameRef.current !== null) {
+      window.cancelAnimationFrame(targetScrollFrameRef.current);
+      targetScrollFrameRef.current = null;
+    }
   };
 
   useEffect(() => {
     handledTargetRef.current = "";
     clearTargetScrollTimers();
-  }, [targetPostId]);
+    setIsAligningQueryTarget(Boolean(queryTargetPostId));
+  }, [queryTargetPostId, targetPostId]);
 
   useEffect(() => clearTargetScrollTimers, []);
 
+  useEffect(() => {
+    if (!isAligningQueryTarget) return;
+    const preventScroll = (event: Event) => event.preventDefault();
+    const preventScrollKeys = (event: KeyboardEvent) => {
+      if (
+        [
+          "ArrowDown",
+          "ArrowUp",
+          "PageDown",
+          "PageUp",
+          "Home",
+          "End",
+          " ",
+        ].includes(event.key)
+      ) {
+        event.preventDefault();
+      }
+    };
+    const html = document.documentElement;
+    const previousOverscroll = html.style.overscrollBehavior;
+    html.style.overscrollBehavior = "none";
+    window.addEventListener("wheel", preventScroll, {
+      capture: true,
+      passive: false,
+    });
+    window.addEventListener("touchmove", preventScroll, {
+      capture: true,
+      passive: false,
+    });
+    window.addEventListener("keydown", preventScrollKeys, { capture: true });
+    return () => {
+      html.style.overscrollBehavior = previousOverscroll;
+      window.removeEventListener("wheel", preventScroll, { capture: true });
+      window.removeEventListener("touchmove", preventScroll, { capture: true });
+      window.removeEventListener("keydown", preventScrollKeys, {
+        capture: true,
+      });
+    };
+  }, [isAligningQueryTarget]);
+
   useEffect(() => {
     if (!targetPostId || handledTargetRef.current === targetPostId) return;
 
@@ -104,31 +153,70 @@ export function MessageStream({ scope }: MessageStreamProps) {
       handledTargetRef.current = targetPostId;
       clearTargetScrollTimers();
 
-      const scrollToTarget = (behavior: ScrollBehavior = "auto") => {
+      const targetScrollTop = () => {
         const target = document.getElementById(`post-${targetPostId}`);
-        if (!target) return;
+        if (!target) return null;
         const filterBottom =
           filterBarRef.current?.getBoundingClientRect().bottom ?? 0;
         const targetTop = target.getBoundingClientRect().top + window.scrollY;
-        window.scrollTo({
+        return Math.max(0, targetTop - filterBottom - 12);
-          top: Math.max(0, targetTop - filterBottom - 12),
+      };
-          left: 0,
+
-          behavior,
+      const scrollToTarget = (behavior: ScrollBehavior = "auto") => {
-        });
+        const top = targetScrollTop();
+        if (top === null) return;
+        window.scrollTo({ top, left: 0, behavior });
+      };
+
+      const boundedSmoothScrollToTarget = () => {
+        const firstTarget = targetScrollTop();
+        if (firstTarget === null) return;
+        const start = window.scrollY;
+        const startedAt = performance.now();
+        const duration = 520;
+        const direction = firstTarget >= start ? 1 : -1;
+        const easeOutCubic = (x: number) => 1 - Math.pow(1 - x, 3);
+
+        const tick = (now: number) => {
+          const latestTarget = targetScrollTop();
+          if (latestTarget === null) return;
+          const progress = Math.min(1, (now - startedAt) / duration);
+          const ideal = start + (latestTarget - start) * easeOutCubic(progress);
+          const next =
+            direction >= 0
+              ? Math.min(ideal, latestTarget)
+              : Math.max(ideal, latestTarget);
+          window.scrollTo({ top: next, left: 0, behavior: "auto" });
+
+          if (progress < 1) {
+            targetScrollFrameRef.current = window.requestAnimationFrame(tick);
+            return;
+          }
+
+          scrollToTarget("auto");
+          setIsAligningQueryTarget(false);
+          targetScrollFrameRef.current = null;
+        };
+
+        targetScrollFrameRef.current = window.requestAnimationFrame(tick);
       };
 
       const prefersReducedMotion = window.matchMedia(
         "(prefers-reduced-motion: reduce)",
       ).matches;
 
-      // Query deep-links (`?post=<id>`) usually come from Home cards/list rows.
+      if (queryTargetPostId) {
-      // Keep that navigation stable by jumping directly to the target instead
+        // Query deep-links (`?post=<id>`) usually come from Home cards/list
-      // of first resetting to the top and then animating through the stream.
+        // rows. Keep the premium motion, but drive it ourselves so scrolling is
-      // Legacy hash links can still use the visible smooth scroll.
+        // bounded to the current target position and cannot visibly pass the
+        // target before snapping back. User-driven scroll is temporarily locked
+        // by the effect above; programmatic scroll remains allowed.
+        targetScrollTimersRef.current = [80].map((ms) =>
+          window.setTimeout(boundedSmoothScrollToTarget, ms),
+        );
+      } else {
         window.requestAnimationFrame(() =>
-        scrollToTarget(
+          scrollToTarget(prefersReducedMotion ? "auto" : "smooth"),
-          queryTargetPostId || prefersReducedMotion ? "auto" : "smooth",
-        ),
         );
 
         // Media above the target can finish loading after the first scroll and
@@ -137,6 +225,7 @@ export function MessageStream({ scope }: MessageStreamProps) {
         targetScrollTimersRef.current = [900, 1400, 2000].map((ms) =>
           window.setTimeout(() => scrollToTarget("auto"), ms),
         );
+      }
 
       el.classList.add("ark-bubble-highlight");
       window.setTimeout(
@@ -148,6 +237,7 @@ export function MessageStream({ scope }: MessageStreamProps) {
 
     // Not loaded yet — keep paging until it appears or the stream is exhausted.
     if (hasMore && !isLoading) loadMore();
+    else if (!hasMore && !isLoading) setIsAligningQueryTarget(false);
   }, [targetPostId, items, hasMore, isLoading, loadMore]);
 
   const updateParam = (key: string, value: string) => {