diff --git a/crates/agent-gui/test/chat/scroll-follow-core.test.mjs b/crates/agent-gui/test/chat/scroll-follow-core.test.mjs
index 213a24e5d..4afa734e1 100644
--- a/crates/agent-gui/test/chat/scroll-follow-core.test.mjs
+++ b/crates/agent-gui/test/chat/scroll-follow-core.test.mjs
@@ -8,6 +8,7 @@ const {
   DEFAULT_FOLLOW_CONFIG,
   createFollowState,
   isDominantVerticalWheel,
+  isPointInNativeScrollbarGutter,
   reduceFollowEvent,
 } = createTsModuleLoader().loadModule("@liveagent/ui/lib/chat-scroll/scrollFollowCore");
 
@@ -188,6 +189,75 @@ test("a scrollbar track click detaches while following", () => {
   assert.equal(pin, false);
 });
 
+test("native scrollbar thumb drag detaches through scroll events alone", () => {
+  // A native thumb drag delivers no pointermove to the page (Chromium/WebKit
+  // route it to the scrollbar), only scroll events. Without the pointerdown
+  // promotion the drag reads as "following + gap opened" and the corrector
+  // re-pins every frame — the thumb looks glued to the bottom. With the
+  // promotion the first away-movement detaches and the drag stays free.
+  const glued = run([growth(0), { type: "pointerDown" }, scroll(120, 10), scroll(260, 20)]);
+  assert.equal(glued.state.following, true, "precondition: unpromoted press gets corrected");
+  assert.equal(glued.pin, true);
+
+  const drag = run([
+    growth(0),
+    { type: "pointerDown" },
+    { type: "pointerDragStart" }, // hook: press landed in the native scrollbar gutter
+    scroll(120, 10),
+    scroll(260, 20),
+    scroll(800, 30),
+  ]);
+  assert.equal(drag.state.following, false);
+  assert.equal(drag.pin, false);
+  // Releasing mid-history stays detached; dragging back down and releasing
+  // inside the zone re-engages exactly like a custom-scrollbar drag.
+  const releasedHigh = run([{ type: "pointerRelease", gap: 800 }], { state: drag.state });
+  assert.equal(releasedHigh.state.following, false);
+  const releasedLow = run([scroll(300, 40), scroll(40, 50), { type: "pointerRelease", gap: 40 }], {
+    state: drag.state,
+  });
+  assert.equal(releasedLow.state.following, true);
+  assert.equal(releasedLow.pin, true);
+});
+
+test("native scrollbar gutter hit test: border box minus client box", () => {
+  // 300x400 border box at (100,50); a 6px vertical scrollbar on the right
+  // (clientWidth 294) and a 6px horizontal one at the bottom (clientHeight 394).
+  const box = {
+    left: 100,
+    top: 50,
+    width: 300,
+    height: 400,
+    clientLeft: 0,
+    clientTop: 0,
+    clientWidth: 294,
+    clientHeight: 394,
+  };
+  // Content presses.
+  assert.equal(isPointInNativeScrollbarGutter(100, 50, box), false);
+  assert.equal(isPointInNativeScrollbarGutter(250, 200, box), false);
+  assert.equal(isPointInNativeScrollbarGutter(393.9, 443.9, box), false);
+  // Vertical scrollbar column, including a `scrollbar-gutter: stable` reserve
+  // (the client box already excludes it).
+  assert.equal(isPointInNativeScrollbarGutter(394, 200, box), true);
+  assert.equal(isPointInNativeScrollbarGutter(399, 200, box), true);
+  // Horizontal scrollbar row.
+  assert.equal(isPointInNativeScrollbarGutter(200, 444, box), true);
+  // Outside the element entirely (a press on a sibling that bubbled through
+  // a listener root) is never a scrollbar press.
+  assert.equal(isPointInNativeScrollbarGutter(400, 200, box), false);
+  assert.equal(isPointInNativeScrollbarGutter(99, 200, box), false);
+  assert.equal(isPointInNativeScrollbarGutter(200, 450, box), false);
+  // No scrollbar at all (clientWidth == width): nothing qualifies.
+  const noBar = { ...box, clientWidth: 300, clientHeight: 400 };
+  assert.equal(isPointInNativeScrollbarGutter(399, 449, noBar), false);
+  // RTL / left-side scrollbar: clientLeft carries the bar width.
+  const rtl = { ...box, clientLeft: 6, clientWidth: 294, clientHeight: 400 };
+  assert.equal(isPointInNativeScrollbarGutter(103, 200, rtl), true);
+  assert.equal(isPointInNativeScrollbarGutter(106, 200, rtl), false);
+  assert.equal(isPointInNativeScrollbarGutter(399, 200, rtl), false);
+});
+
 test("held pointer suppresses zone attach; release inside the zone re-engages", () => {
   const midDrag = run([
     wheelUp({ now: 0 }),
diff --git a/crates/agent-gui/test/chat/use-scroll-follow-native-scrollbar.test.mjs b/crates/agent-gui/test/chat/use-scroll-follow-native-scrollbar.test.mjs
new file mode 100644
index 000000000..5d0ee3bc0
--- /dev/null
+++ b/crates/agent-gui/test/chat/use-scroll-follow-native-scrollbar.test.mjs
@@ -0,0 +1,192 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+import { createDomTestEnv } from "../helpers/dom-test-env.mjs";
+
+// Regression for the "scrollbar thumb glued to the bottom" bug: the transcript
+// (GUI) and the Base UI viewport (WebUI, native bar hidden but still the
+// scroller) both run useScrollFollow on a native-scrollbar viewport. A native
+// thumb drag emits scroll events only — no pointermove reaches the page — so
+// the hook must promote a press inside the scrollbar gutter to a drag on
+// pointerdown, otherwise the corrector re-pins on every drag frame.
+
+const env = await createDomTestEnv();
+const { useScrollFollow } = env.loadModule("@liveagent/ui/lib/chat-scroll/useScrollFollow");
+const React = env.React;
+
+// jsdom has no layout: fake a 300x400 viewport with a 6px vertical scrollbar
+// and 2000px of content, and make scrollTop writable + scroll-event driven.
+function installGeometry(el, { width = 300, height = 400, barWidth = 6, scrollHeight = 2000 }) {
+  let scrollTop = 0;
+  Object.defineProperties(el, {
+    clientWidth: { value: width - barWidth, configurable: true },
+    clientHeight: { value: height, configurable: true },
+    clientLeft: { value: 0, configurable: true },
+    clientTop: { value: 0, configurable: true },
+    scrollHeight: { value: scrollHeight, configurable: true },
+    scrollTop: {
+      configurable: true,
+      get: () => scrollTop,
+      set: (value) => {
+        scrollTop = Math.max(0, Math.min(scrollHeight - height, value));
+      },
+    },
+  });
+  el.getBoundingClientRect = () => ({
+    left: 100,
+    top: 50,
+    width,
+    height,
+    right: 100 + width,
+    bottom: 50 + height,
+    x: 100,
+    y: 50,
+    toJSON() {},
+  });
+}
+
+function pointer(type, target, init) {
+  const Ctor = env.dom.window.PointerEvent ?? env.dom.window.MouseEvent;
+  const event = new Ctor(type, { bubbles: true, cancelable: true, ...init });
+  if (!("pointerType" in event) || event.pointerType === undefined) {
+    Object.defineProperty(event, "pointerType", { value: init.pointerType ?? "mouse" });
+  }
+  if (event.button === undefined || event.button !== (init.button ?? 0)) {
+    Object.defineProperty(event, "button", { value: init.button ?? 0 });
+  }
+  if (init.buttons !== undefined && event.buttons !== init.buttons) {
+    Object.defineProperty(event, "buttons", { value: init.buttons });
+  }
+  target.dispatchEvent(event);
+  return event;
+}
+
+function mount() {
+  const container = env.dom.window.document.createElement("div");
+  env.dom.window.document.body.appendChild(container);
+  const root = env.createRoot(container);
+  const captured = { handle: null, following: null, viewport: null };
+
+  function Harness() {
+    const [viewport, setViewport] = React.useState(null);
+    const setRef = React.useCallback((el) => {
+      if (el) installGeometry(el, {});
+      setViewport(el);
+    }, []);
+    const { handle, following } = useScrollFollow({
+      viewport,
+      listenerRoot: viewport,
+    });
+    captured.handle = handle;
+    captured.following = following;
+    captured.viewport = viewport;
+    return React.createElement(
+      "div",
+      { ref: setRef, "data-scroll-viewport": "" },
+      React.createElement("div", null, "content"),
+    );
+  }
+
+  env.act(() => {
+    root.render(React.createElement(Harness));
+  });
+  return { root, container, captured };
+}
+
+// Simulate a native thumb drag: the browser moves scrollTop and emits scroll
+// events; no pointermove reaches the page.
+function nativeDragTo(viewport, ...tops) {
+  for (const top of tops) {
+    env.act(() => {
+      viewport.scrollTop = top;
+      viewport.dispatchEvent(new env.dom.window.Event("scroll"));
+    });
+  }
+}
+
+test("a mouse press inside the native scrollbar gutter lets a thumb drag detach follow", () => {
+  const { root, captured } = mount();
+  const viewport = captured.viewport;
+  assert.ok(viewport);
+  assert.equal(captured.following, true);
+  // Mount pin.
+  assert.equal(viewport.scrollTop, 1600);
+
+  // Press at x=397: inside the 300px border box (100..400) but past the
+  // 294px client box — the vertical scrollbar column.
+  env.act(() => {
+    pointer("pointerdown", viewport, { clientX: 397, clientY: 200, pointerType: "mouse" });
+  });
+  nativeDragTo(viewport, 1400, 1100, 600);
+
+  assert.equal(captured.following, false, "thumb drag must detach follow");
+  assert.equal(viewport.scrollTop, 600, "drag position must not be corrected back to the bottom");
+
+  // Release far from the bottom stays detached.
+  env.act(() => {
+    pointer("pointerup", env.dom.window, { clientX: 397, clientY: 120, pointerType: "mouse" });
+  });
+  assert.equal(captured.following, false);
+  assert.equal(viewport.scrollTop, 600);
+  env.act(() => root.unmount());
+});
+
+test("dragging the native thumb back to the bottom and releasing re-engages follow", () => {
+  const { root, captured } = mount();
+  const viewport = captured.viewport;
+
+  env.act(() => {
+    pointer("pointerdown", viewport, { clientX: 397, clientY: 200, pointerType: "mouse" });
+  });
+  nativeDragTo(viewport, 900, 300);
+  assert.equal(captured.following, false);
+  nativeDragTo(viewport, 1200, 1550);
+  // Still held: no attach mid-drag.
+  assert.equal(captured.following, false);
+  env.act(() => {
+    pointer("pointerup", env.dom.window, { clientX: 397, clientY: 380, pointerType: "mouse" });
+  });
+  assert.equal(captured.following, true);
+  assert.equal(viewport.scrollTop, 1600, "release inside the zone pins to the bottom");
+  env.act(() => root.unmount());
+});
+
+test("a static content press followed by a layout echo is still corrected", () => {
+  // The slop gate for content clicks is unchanged: a press inside the client
+  // box never counts as a scrollbar press, so a scroll event that opens a gap
+  // with no drag is treated as noise and re-pinned.
+  const { root, captured } = mount();
+  const viewport = captured.viewport;
+
+  env.act(() => {
+    pointer("pointerdown", viewport, { clientX: 200, clientY: 200, pointerType: "mouse" });
+  });
+  nativeDragTo(viewport, 1500);
+  assert.equal(captured.following, true);
+  assert.equal(viewport.scrollTop, 1600, "content press + gap is corrected back to the bottom");
+  env.act(() => {
+    pointer("pointerup", env.dom.window, { clientX: 200, clientY: 200, pointerType: "mouse" });
+  });
+  env.act(() => root.unmount());
+});
+
+test("a touch press in the gutter column is not a scrollbar press", () => {
+  // Touch never grabs a native thumb; a finger landing over the gutter is a
+  // content touch and goes through the touchmove path instead.
+  const { root, captured } = mount();
+  const viewport = captured.viewport;
+
+  env.act(() => {
+    pointer("pointerdown", viewport, { clientX: 397, clientY: 200, pointerType: "touch" });
+  });
+  nativeDragTo(viewport, 1500);
+  assert.equal(captured.following, true);
+  assert.equal(viewport.scrollTop, 1600);
+  env.act(() => {
+    pointer("pointerup", env.dom.window, { clientX: 397, clientY: 200, pointerType: "touch" });
+  });
+  env.act(() => root.unmount());
+});
+
+test.after(() => {
+  env.cleanup();
+});
diff --git a/crates/agent-ui/src/lib/chat-scroll/scrollFollowCore.ts b/crates/agent-ui/src/lib/chat-scroll/scrollFollowCore.ts
index 9f42d9250..0422d3198 100644
--- a/crates/agent-ui/src/lib/chat-scroll/scrollFollowCore.ts
+++ b/crates/agent-ui/src/lib/chat-scroll/scrollFollowCore.ts
@@ -60,6 +60,48 @@ export const DIRECTION_SLOP_PX = 1;
 // can never read as "dragged away from the bottom".
 export const POINTER_DRAG_SLOP_PX = 4;
 
+// Geometry of a scroll container for native-scrollbar hit testing, all in the
+// same CSS-px coordinate space (getBoundingClientRect + client* metrics).
+export type ScrollerBox = {
+  // Border box (getBoundingClientRect).
+  left: number;
+  top: number;
+  width: number;
+  height: number;
+  // Client box offset from the border box origin (clientLeft/clientTop). In
+  // RTL clientLeft already includes a left-side vertical scrollbar.
+  clientLeft: number;
+  clientTop: number;
+  // Client box size (clientWidth/clientHeight) — excludes scrollbars and any
+  // `scrollbar-gutter: stable` reserve.
+  clientWidth: number;
+  clientHeight: number;
+};
+
+// A press inside the border box but outside the client box landed on the
+// element's own native scrollbar (thumb, track or reserved gutter). Native
+// thumb drags deliver no pointermove to the page — Chromium and WebKit route
+// them to the scrollbar — only scroll events, so the movement-slop promotion
+// in the hook can never fire; the hook must promote such a press to a drag
+// on pointerdown, exactly like a press on a custom scrollbar element.
+// Border pixels also read as "scrollbar": the scrollers using this engine
+// have no borders, and a press on a border is never content interaction.
+export function isPointInNativeScrollbarGutter(x: number, y: number, box: ScrollerBox) {
+  const insideBorderBox =
+    x >= box.left && x < box.left + box.width && y >= box.top && y < box.top + box.height;
+  if (!insideBorderBox) {
+    return false;
+  }
+  const clientX0 = box.left + box.clientLeft;
+  const clientY0 = box.top + box.clientTop;
+  const insideClientBox =
+    x >= clientX0 &&
+    x < clientX0 + box.clientWidth &&
+    y >= clientY0 &&
+    y < clientY0 + box.clientHeight;
+  return !insideClientBox;
+}
+
 // Elements carrying this attribute keep their arrow/Home/End keys to
 // themselves: the transcript width handles resize on those keys, which must
 // not also read as a scroll intent and detach bottom-follow. useScrollFollow
diff --git a/crates/agent-ui/src/lib/chat-scroll/useScrollFollow.ts b/crates/agent-ui/src/lib/chat-scroll/useScrollFollow.ts
index baa7bfe4e..145e69ba2 100644
--- a/crates/agent-ui/src/lib/chat-scroll/useScrollFollow.ts
+++ b/crates/agent-ui/src/lib/chat-scroll/useScrollFollow.ts
@@ -7,6 +7,7 @@ import {
   type FollowEvent,
   type FollowState,
   isDominantVerticalWheel,
+  isPointInNativeScrollbarGutter,
   POINTER_DRAG_SLOP_PX,
   reduceFollowEvent,
   SCROLL_FOLLOW_IGNORE_KEYS_ATTRIBUTE,
@@ -289,6 +290,26 @@ export function useScrollFollow(args: UseScrollFollowArgs): {
     // can swallow the matching pointerup.
     let pointerDownX = 0;
     let pointerDownY = 0;
+    // The viewport's own native scrollbar has no DOM node to hit-test, so a
+    // press is classified geometrically: inside the border box but outside
+    // the client box. Only mouse presses qualify — touch scrolls the content
+    // and never grabs a native thumb, and a stylus press is not a scroll intent.
+    const isPressOnNativeScrollbar = (event: PointerEvent) => {
+      if (event.pointerType !== "mouse" || event.target !== viewport) {
+        return false;
+      }
+      const rect = viewport.getBoundingClientRect();
+      return isPointInNativeScrollbarGutter(event.clientX, event.clientY, {
+        left: rect.left,
+        top: rect.top,
+        width: rect.width,
+        height: rect.height,
+        clientLeft: viewport.clientLeft,
+        clientTop: viewport.clientTop,
+        clientWidth: viewport.clientWidth,
+        clientHeight: viewport.clientHeight,
+      });
+    };
     const handlePointerDown = (event: PointerEvent) => {
       if (event.pointerType === "mouse" && event.button === 2) {
         return;
@@ -296,12 +317,21 @@ export function useScrollFollow(args: UseScrollFollowArgs): {
       pointerDownX = event.clientX;
       pointerDownY = event.clientY;
       dispatch({ type: "pointerDown" });
-      // A press on the custom scrollbar is unambiguous scroll intent, and a
-      // track click jumps scrollTop synchronously on pointerdown with zero
-      // pointer movement — the movement-slop promotion below would never fire
-      // and the corrector would undo the jump. Content clicks keep the slop
-      // gate (static click + layout echo must not read as a drag).
-      if (event.target instanceof Element && event.target.closest("[data-scroll-area-scrollbar]")) {
+      // A press on a scrollbar is unambiguous scroll intent, and neither kind
+      // can rely on the movement-slop promotion below:
+      // - a custom-scrollbar track click jumps scrollTop synchronously on
+      //   pointerdown with zero pointer movement, so the corrector would undo
+      //   the jump;
+      // - a native thumb drag delivers no pointermove to the page at all
+      //   (Chromium/WebKit route it to the scrollbar), only scroll events, so
+      //   the drag would read as "following + gap opened" and get re-pinned to
+      //   the bottom on every frame — the thumb looks glued in place.
+      // Content clicks keep the slop gate (static click + layout echo must not
+      // read as a drag).
+      if (
+        (event.target instanceof Element && event.target.closest("[data-scroll-area-scrollbar]")) ||
+        isPressOnNativeScrollbar(event)
+      ) {
         cancelJumpAnimation();
         dispatch({ type: "pointerDragStart" });
       }