feat(thread-drawer): wrap in horseshoe seam with rounded TL/BL and add pointer/keyboard resize clamped to viewport/3

src/app/features/room/Room.tsx

@@ -122,7 +122,11 @@ export function Room({ renderRoomView }: RoomProps) {
   // a time; both feed into `showAnyHorseshoe` for the chat-column
   // bg + the void-gap render gate.
   const showMediaHorseshoe = mediaOpen && !isMobile && !showThreadDrawer;
-  const showAnyHorseshoe = showProfileHorseshoe || showMediaHorseshoe;
+  // Thread drawer side pane on desktop. Mobile hides the chat column
+  // entirely (`drawerHidesChat`) so the seam doesn't apply there.
+  const showThreadHorseshoe = showThreadDrawer && !isMobile;
+  const showAnyHorseshoe =
+    showProfileHorseshoe || showMediaHorseshoe || showThreadHorseshoe;
 
   useKeyDown(
     window,
@@ -278,13 +282,24 @@ export function Room({ renderRoomView }: RoomProps) {
               </>
             )}
           {showThreadDrawer && decodedRootId && parentRoomPath && (
-            <ThreadDrawer
-              key={`${room.roomId}/${decodedRootId}`}
-              room={room}
-              rootId={decodedRootId}
-              parentRoomPath={parentRoomPath}
-              variant={isMobile ? 'mobile' : 'desktop'}
-            />
+            <>
+              {showThreadHorseshoe && (
+                <Box
+                  shrink="No"
+                  style={{
+                    width: toRem(VOJO_HORSESHOE_GAP_PX),
+                    backgroundColor: VOJO_HORSESHOE_VOID_COLOR,
+                  }}
+                />
+              )}
+              <ThreadDrawer
+                key={`${room.roomId}/${decodedRootId}`}
+                room={room}
+                rootId={decodedRootId}
+                parentRoomPath={parentRoomPath}
+                variant={isMobile ? 'mobile' : 'desktop'}
+              />
+            </>
           )}
         </Box>
         </MediaViewerHostContext.Provider>

src/app/features/room/ThreadDrawer.css.ts

@@ -1,5 +1,80 @@
 import { style } from '@vanilla-extract/css';
 import { color, config, toRem } from 'folds';
+import {
+  VOJO_HORSESHOE_GAP_PX,
+  VOJO_HORSESHOE_RADIUS_PX,
+} from '../../styles/horseshoe';
+
+// Desktop wrapper for the resizable thread drawer. Sizing and the
+// absolutely-positioned resize handle live here; the inner aside
+// (`ThreadDrawer` below) owns the rounded TL/BL carves + bg.
+// `overflow: visible` on the wrapper is critical — the handle sits
+// at `left: -GAP_PX` inside the horseshoe void, and the aside's
+// `overflow: hidden` would otherwise clip it.
+export const ThreadDrawerResizable = style({
+  position: 'relative',
+  flexShrink: 0,
+  flexGrow: 0,
+  maxHeight: '100%',
+  minHeight: 0,
+  display: 'flex',
+  flexDirection: 'column',
+});
+
+// Resize handle mirror of `PageNavResizeHandle` — sits in the 12px
+// horseshoe void to the LEFT of the drawer (page-nav's handle sits in
+// the gap to the right of the nav). Same cosmetics: thin indicator bar
+// that fades in on hover/focus and stretches/shrinks at the clamps.
+export const ThreadDrawerResizeHandle = style({
+  position: 'absolute',
+  top: 0,
+  bottom: 0,
+  left: `-${toRem(VOJO_HORSESHOE_GAP_PX)}`,
+  width: toRem(VOJO_HORSESHOE_GAP_PX),
+  cursor: 'col-resize',
+  zIndex: 1,
+  background: 'transparent',
+  touchAction: 'none',
+  outline: 'none',
+  selectors: {
+    '&::before': {
+      content: '""',
+      position: 'absolute',
+      top: '50%',
+      left: '50%',
+      width: 2,
+      height: toRem(36),
+      transform: 'translate(-50%, -50%)',
+      borderRadius: 1,
+      backgroundColor: color.Surface.OnContainer,
+      opacity: 0,
+      transition:
+        'opacity 140ms ease, height 140ms ease, width 140ms ease, background-color 140ms ease',
+    },
+    '&:hover::before, &:focus-visible::before': {
+      opacity: 0.25,
+    },
+    '&:focus-visible::before': {
+      backgroundColor: color.Primary.Main,
+      opacity: 0.45,
+    },
+    '&[data-dragging="true"]::before': {
+      opacity: 0.55,
+      height: toRem(48),
+      backgroundColor: color.Primary.Main,
+    },
+    '&[data-dragging="true"][data-at-min="true"]::before': {
+      height: toRem(28),
+      width: 3,
+      opacity: 0.85,
+    },
+    '&[data-dragging="true"][data-at-max="true"]::before': {
+      height: toRem(76),
+      width: 2,
+      opacity: 0.9,
+    },
+  },
+});
 
 // Layout copies element-web `_ThreadPanel.pcss:73-84`:
 // `max-height: 100%` clamps the column to the parent's viewport-bound
@@ -7,15 +82,25 @@ import { color, config, toRem } from 'folds';
 // under content (otherwise flex-children grow with content and push
 // the composer off-screen — bug observed on cold-load with many
 // replies). Comment in element-web: «don't displace the composer».
+//
+// Horseshoe: rounded TL/BL mirrors the page-nav <-> chat and chat <->
+// profile/media seams. Room.tsx paints the 12px void gap to the left
+// of the drawer (`showThreadHorseshoe`) and the chat-column gets an
+// explicit Background bg so the void can't bleed through.
+//
+// Sizing (`width`, `flexShrink`) was moved to `ThreadDrawerResizable`
+// so the aside fills its resizable wrapper and the wrapper owns the
+// flex-row contract with Room.tsx.
 export const ThreadDrawer = style({
-  flexShrink: 0,
-  width: `clamp(${toRem(320)}, 28%, ${toRem(420)})`,
-  maxHeight: '100%',
+  flexGrow: 1,
+  width: '100%',
   display: 'flex',
   flexDirection: 'column',
   backgroundColor: color.Surface.Container,
-  borderLeft: `${config.borderWidth.B300} solid ${color.Surface.ContainerLine}`,
   minHeight: 0,
+  overflow: 'hidden',
+  borderTopLeftRadius: toRem(VOJO_HORSESHOE_RADIUS_PX),
+  borderBottomLeftRadius: toRem(VOJO_HORSESHOE_RADIUS_PX),
 });
 
 // Mobile push: drawer occupies the whole content column, not a side pane.

src/app/features/room/ThreadDrawer.tsx

@@ -1,7 +1,7 @@
 import React, { useCallback, useEffect, useLayoutEffect, useMemo, useRef, useState } from 'react';
 import { useNavigate, useParams } from 'react-router-dom';
 import { useTranslation } from 'react-i18next';
-import { useAtomValue, useSetAtom } from 'jotai';
+import { useAtom, useAtomValue, useSetAtom } from 'jotai';
 import {
   Box,
   Button,
@@ -13,6 +13,7 @@ import {
   Spinner,
   Text,
   config,
+  toRem,
 } from 'folds';
 import {
   Direction,
@@ -82,6 +83,11 @@ import { roomToParentsAtom } from '../../state/room/roomToParents';
 import { draftKey, roomIdToReplyDraftAtomFamily } from '../../state/room/roomInputDrafts';
 import { useSetting } from '../../state/hooks/settings';
 import { settingsAtom } from '../../state/settings';
+import {
+  THREAD_DRAWER_WIDTH_MIN,
+  clampThreadDrawerWidth,
+  threadDrawerWidthAtom,
+} from '../../state/threadDrawerWidth';
 import {
   factoryRenderLinkifyWithMention,
   getReactCustomHtmlParser,
@@ -154,6 +160,130 @@ export function ThreadDrawer({
   const editor = useEditor();
   const fileDropContainerRef = useRef<HTMLDivElement>(null);
   const closeBtnRef = useRef<HTMLButtonElement>(null);
+
+  // Desktop-only resizable side pane. Mirrors `ResizablePageNav` in
+  // `components/page/Page.tsx`: width persisted to localStorage via
+  // `threadDrawerWidthAtom`, clamped [MIN, viewport/3]. Mobile variant
+  // doesn't render the wrapper at all so these hooks are effectively
+  // no-ops there — they always run (rules-of-hooks) but the ref / DOM
+  // is never wired up.
+  const isDesktopVariant = variant === 'desktop';
+  const resizableRef = useRef<HTMLDivElement>(null);
+  const resizeHandleRef = useRef<HTMLDivElement>(null);
+  const [savedWidth, setSavedWidth] = useAtom(threadDrawerWidthAtom);
+  const [vw, setVw] = useState<number>(
+    typeof window !== 'undefined' ? window.innerWidth : 1280
+  );
+  const [dragging, setDragging] = useState(false);
+  // Live width during a drag — kept in component state so we don't
+  // flush localStorage on every pointermove (hundreds of sync disk
+  // writes per drag). Atom is committed once on pointerup.
+  const [liveWidth, setLiveWidth] = useState<number | null>(null);
+
+  useEffect(() => {
+    if (!isDesktopVariant) return undefined;
+    const onResize = () => setVw(window.innerWidth);
+    window.addEventListener('resize', onResize);
+    return () => window.removeEventListener('resize', onResize);
+  }, [isDesktopVariant]);
+
+  const drawerMaxW = Math.max(THREAD_DRAWER_WIDTH_MIN, Math.floor(vw / 3));
+  const drawerBaseWidth = dragging && liveWidth !== null ? liveWidth : savedWidth;
+  const drawerWidth = clampThreadDrawerWidth(drawerBaseWidth, vw);
+  // Viewports too narrow for any meaningful range (max == min) hide the
+  // handle entirely — leaving a non-draggable handle reads as broken UI.
+  const canResize = isDesktopVariant && drawerMaxW > THREAD_DRAWER_WIDTH_MIN;
+  const atMin =
+    dragging && liveWidth !== null && liveWidth <= THREAD_DRAWER_WIDTH_MIN;
+  const atMax = dragging && liveWidth !== null && liveWidth >= drawerMaxW;
+
+  // Body-style cleanup if the drag terminates without `pointerup`
+  // (component unmount mid-drag — route change, mobile breakpoint flip,
+  // Alt-Tab). Without this the page can get stuck with col-resize cursor
+  // + userSelect: none.
+  useEffect(() => {
+    if (!dragging) return undefined;
+    return () => {
+      document.body.style.cursor = '';
+      document.body.style.userSelect = '';
+    };
+  }, [dragging]);
+
+  const beginDrag = useCallback((pointerId: number) => {
+    setDragging(true);
+    setLiveWidth(null);
+    try {
+      resizeHandleRef.current?.setPointerCapture(pointerId);
+    } catch {
+      /* setPointerCapture is best-effort */
+    }
+    document.body.style.cursor = 'col-resize';
+    document.body.style.userSelect = 'none';
+  }, []);
+
+  const endDrag = useCallback(() => {
+    setDragging(false);
+    document.body.style.cursor = '';
+    document.body.style.userSelect = '';
+    setLiveWidth((current) => {
+      if (current !== null) {
+        setSavedWidth(clampThreadDrawerWidth(current, window.innerWidth));
+      }
+      return null;
+    });
+  }, [setSavedWidth]);
+
+  const onResizePointerDown = useCallback(
+    (e: React.PointerEvent<HTMLDivElement>) => {
+      if (e.button !== 0 || !canResize) return;
+      e.preventDefault();
+      beginDrag(e.pointerId);
+    },
+    [beginDrag, canResize]
+  );
+
+  const onResizePointerMove = useCallback(
+    (e: React.PointerEvent<HTMLDivElement>) => {
+      if (!dragging || !resizableRef.current) return;
+      // Right edge of the wrapper is anchored at the row's right edge
+      // (drawer is the last item in Room.tsx's flex row). Width grows
+      // as the pointer moves LEFT, shrinks as it moves right — inverse
+      // of the page-nav handle.
+      const rect = resizableRef.current.getBoundingClientRect();
+      setLiveWidth(clampThreadDrawerWidth(rect.right - e.clientX, window.innerWidth));
+    },
+    [dragging]
+  );
+
+  const onResizeStopPointer = useCallback(
+    (e: React.PointerEvent<HTMLDivElement>) => {
+      try {
+        resizeHandleRef.current?.releasePointerCapture(e.pointerId);
+      } catch {
+        /* releasePointerCapture is best-effort */
+      }
+      endDrag();
+    },
+    [endDrag]
+  );
+
+  const onResizeKeyDown = useCallback(
+    (e: React.KeyboardEvent<HTMLDivElement>) => {
+      if (!canResize) return;
+      const step = e.shiftKey ? 64 : 16;
+      let next: number | null = null;
+      // Arrow direction is inverted vs the page-nav handle: ArrowLeft
+      // grows the drawer (its left edge moves left → width increases).
+      if (e.key === 'ArrowLeft') next = savedWidth + step;
+      else if (e.key === 'ArrowRight') next = savedWidth - step;
+      else if (e.key === 'Home') next = drawerMaxW;
+      else if (e.key === 'End') next = THREAD_DRAWER_WIDTH_MIN;
+      if (next === null) return;
+      e.preventDefault();
+      setSavedWidth(clampThreadDrawerWidth(next, vw));
+    },
+    [savedWidth, vw, drawerMaxW, canResize, setSavedWidth]
+  );
   // Sentinel at the bottom of the replies list. The autoscroll effect
   // below scrolls into view on (a) initial reveal, (b) reply count
   // growth while at-bottom, (c) own send. Whether we're at-bottom is
@@ -1136,10 +1266,10 @@ export function ThreadDrawer({
     );
   };
 
-  return (
+  const asideContent = (
     <aside
       ref={fileDropContainerRef}
-      className={variant === 'desktop' ? css.ThreadDrawer : css.ThreadDrawerMobile}
+      className={isDesktopVariant ? css.ThreadDrawer : css.ThreadDrawerMobile}
       role="region"
       aria-labelledby={headerId}
     >
@@ -1198,4 +1328,40 @@ export function ThreadDrawer({
       </div>
     </aside>
   );
+
+  if (!isDesktopVariant) {
+    return asideContent;
+  }
+
+  return (
+    <div
+      ref={resizableRef}
+      className={css.ThreadDrawerResizable}
+      style={{ width: drawerWidth }}
+    >
+      {asideContent}
+      {canResize && (
+        <div
+          ref={resizeHandleRef}
+          role="separator"
+          aria-orientation="vertical"
+          aria-valuenow={drawerWidth}
+          aria-valuemin={THREAD_DRAWER_WIDTH_MIN}
+          aria-valuemax={drawerMaxW}
+          aria-label="Resize thread drawer"
+          tabIndex={0}
+          className={css.ThreadDrawerResizeHandle}
+          data-dragging={dragging || undefined}
+          data-at-min={atMin || undefined}
+          data-at-max={atMax || undefined}
+          onPointerDown={onResizePointerDown}
+          onPointerMove={onResizePointerMove}
+          onPointerUp={onResizeStopPointer}
+          onPointerCancel={onResizeStopPointer}
+          onLostPointerCapture={onResizeStopPointer}
+          onKeyDown={onResizeKeyDown}
+        />
+      )}
+    </div>
+  );
 }

src/app/state/threadDrawerWidth.ts

@@ -0,0 +1,33 @@
+import {
+  atomWithLocalStorage,
+  getLocalStorageItem,
+  setLocalStorageItem,
+} from './utils/atomWithLocalStorage';
+
+export const THREAD_DRAWER_WIDTH_KEY = 'vojo_thread_drawer_width';
+// Match the prior static `clamp(320, 28%, 420)` lower bound so users on
+// narrow viewports keep the same minimum they had pre-resize.
+export const THREAD_DRAWER_WIDTH_MIN = 320;
+// Upper end of the prior clamp — pleasant default for typical desktops.
+export const THREAD_DRAWER_WIDTH_DEFAULT = 420;
+
+const readThreadDrawerWidth = (key: string): number => {
+  const raw = getLocalStorageItem<unknown>(key, THREAD_DRAWER_WIDTH_DEFAULT);
+  const value =
+    typeof raw === 'number' && Number.isFinite(raw) ? raw : THREAD_DRAWER_WIDTH_DEFAULT;
+  return Math.max(THREAD_DRAWER_WIDTH_MIN, Math.round(value));
+};
+
+export const threadDrawerWidthAtom = atomWithLocalStorage<number>(
+  THREAD_DRAWER_WIDTH_KEY,
+  readThreadDrawerWidth,
+  setLocalStorageItem
+);
+
+// Symmetric with `clampSidebarWidth` — max = viewport/3 with a MIN floor
+// so single-window-fraction viewports don't collapse the drawer below
+// what's readable.
+export const clampThreadDrawerWidth = (px: number, viewportWidth: number): number => {
+  const max = Math.max(THREAD_DRAWER_WIDTH_MIN, Math.floor(viewportWidth / 3));
+  return Math.max(THREAD_DRAWER_WIDTH_MIN, Math.min(max, Math.round(px)));
+};