fix: reliable window drag region in tab bar

Includes scroll improvements:
- Scroll to bottom on session open and live auto-scroll
- Make auto-scroll StrictMode-safe via needsInitialScrollRef
- Add floating scroll-to-bottom button in chat view

Window drag fix:
- Apply drag region to leftmost pane TabBar regardless of sidebar state
- Cap tab list at 75% so drag spacer always has room
- Add explicit flex-1 drag spacer between tabs and action buttons
- Set WebkitAppRegion: no-drag on tab items for reordering

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/renderer/components/chat/ChatHistory.tsx

@@ -1,11 +1,12 @@
 import { useCallback, useEffect, useMemo, useRef, useState } from 'react';
 
-import { useAutoScrollBottom } from '@renderer/hooks/useAutoScrollBottom';
+import { isNearBottom, useAutoScrollBottom } from '@renderer/hooks/useAutoScrollBottom';
 import { useTabNavigationController } from '@renderer/hooks/useTabNavigationController';
 import { useTabUI } from '@renderer/hooks/useTabUI';
 import { useVisibleAIGroup } from '@renderer/hooks/useVisibleAIGroup';
 import { useStore } from '@renderer/store';
 import { useVirtualizer } from '@tanstack/react-virtual';
+import { ChevronsDown } from 'lucide-react';
 import { useShallow } from 'zustand/react/shallow';
 
 import { SessionContextPanel } from './SessionContextPanel/index';
@@ -343,11 +344,21 @@ export const ChatHistory = ({ tabId }: ChatHistoryProps): JSX.Element => {
     rootRef: scrollContainerRef,
   });
 
+  // Scroll-to-bottom button visibility
+  const [showScrollButton, setShowScrollButton] = useState(false);
+
+  const checkScrollButton = useCallback(() => {
+    const container = scrollContainerRef.current;
+    if (!container) return;
+    const { scrollTop, scrollHeight, clientHeight } = container;
+    setShowScrollButton(!isNearBottom(scrollTop, scrollHeight, clientHeight, 300));
+  }, []);
+
   // Auto-follow when conversation updates, but only if the user was already near bottom.
   // This preserves manual reading position when the user scrolls up.
   // Disabled during navigation to prevent conflicts with deep-link/search scrolling.
-  useAutoScrollBottom([conversation], {
+  const { scrollToBottom } = useAutoScrollBottom([conversation], {
-    threshold: 150,
+    threshold: 300,
     smoothDuration: 300,
     autoBehavior: 'auto',
     disabled: shouldDisableAutoScroll,
@@ -355,6 +366,11 @@ export const ChatHistory = ({ tabId }: ChatHistoryProps): JSX.Element => {
     resetKey: effectiveTabId,
   });
 
+  // Re-check button visibility whenever conversation updates
+  useEffect(() => {
+    checkScrollButton();
+  }, [conversation, checkScrollButton]);
+
   // Callback to register AI group refs (combines with visibility hook)
   const registerAIGroupRefCombined = useCallback(
     (groupId: string) => {
@@ -718,12 +734,13 @@ export const ChatHistory = ({ tabId }: ChatHistoryProps): JSX.Element => {
       className="flex flex-1 flex-col overflow-hidden"
       style={{ backgroundColor: 'var(--color-surface)' }}
     >
-      <div className="flex flex-1 overflow-hidden">
+      <div className="relative flex flex-1 overflow-hidden">
         {/* Chat content */}
         <div
           ref={scrollContainerRef}
           className="flex-1 overflow-y-auto"
           style={{ backgroundColor: 'var(--color-surface)' }}
+          onScroll={checkScrollButton}
         >
           {/* Sticky Context button */}
           {allContextInjections.length > 0 && (
@@ -813,6 +830,30 @@ export const ChatHistory = ({ tabId }: ChatHistoryProps): JSX.Element => {
           </div>
         </div>
 
+        {/* Scroll to bottom button */}
+        {showScrollButton && (
+          <button
+            onClick={() => {
+              scrollToBottom('smooth');
+              setShowScrollButton(false);
+            }}
+            className="absolute bottom-5 z-20 flex items-center gap-1.5 rounded-full px-3 py-1.5 text-xs shadow-lg backdrop-blur-md transition-all"
+            style={{
+              right:
+                isContextPanelVisible && allContextInjections.length > 0
+                  ? 'calc(320px + 1rem)'
+                  : '1rem',
+              backgroundColor: 'var(--context-btn-bg)',
+              color: 'var(--color-text-secondary)',
+              border: '1px solid var(--color-border-emphasis)',
+            }}
+            title="Scroll to bottom"
+          >
+            <ChevronsDown className="size-3.5" />
+            <span>Bottom</span>
+          </button>
+        )}
+
         {/* Context panel sidebar */}
         {isContextPanelVisible && allContextInjections.length > 0 && (
           <div className="w-80 shrink-0">

src/renderer/components/layout/SortableTab.tsx

@@ -60,7 +60,8 @@ export const SortableTab = ({
     },
   });
 
-  const style: React.CSSProperties = {
+  const style = {
+    WebkitAppRegion: 'no-drag',
     transform: CSS.Transform.toString(transform),
     transition: isDragging ? 'none' : transition,
     opacity: isDragging ? 0.3 : 1,

src/renderer/components/layout/TabBar.tsx

@@ -268,8 +268,7 @@ export const TabBar = ({ paneId }: TabBarProps): React.JSX.Element => {
             sidebarCollapsed && isLeftmostPane
               ? 'var(--macos-traffic-light-padding-left, 72px)'
               : '8px',
-          WebkitAppRegion:
+          WebkitAppRegion: isElectronMode() && isLeftmostPane ? 'drag' : undefined,
-            isElectronMode() && sidebarCollapsed && isLeftmostPane ? 'drag' : undefined,
           backgroundColor: 'var(--color-surface)',
           borderBottom: '1px solid var(--color-border)',
           opacity: isFocused || paneCount === 1 ? 1 : 0.7,
@@ -296,15 +295,17 @@ export const TabBar = ({ paneId }: TabBarProps): React.JSX.Element => {
         </button>
       )}
 
-      {/* Tab list with horizontal scroll, sortable DnD, and droppable area */}
+      {/* Tab list with horizontal scroll, sortable DnD, and droppable area.
+          Capped at 75% so the drag spacer always has room to the right. */}
       <div
         ref={(el) => {
           scrollContainerRef.current = el;
           setDroppableRef(el);
         }}
-        className="scrollbar-none flex min-w-0 flex-1 items-center gap-1 overflow-x-auto"
+        className="scrollbar-none flex min-w-0 shrink items-center gap-1 overflow-x-auto"
         style={
           {
+            maxWidth: '75%',
             WebkitAppRegion: 'no-drag',
             outline: isDroppableOver ? '1px dashed var(--color-accent, #6366f1)' : 'none',
             outlineOffset: '-1px',
@@ -346,6 +347,13 @@ export const TabBar = ({ paneId }: TabBarProps): React.JSX.Element => {
         )}
       </div>
 
+      {/* Drag spacer — fills empty space between tab list and action buttons.
+          Gives users a reliable window-drag target regardless of how many tabs are open. */}
+      <div
+        className="flex-1 self-stretch"
+        style={{ WebkitAppRegion: 'drag' } as React.CSSProperties}
+      />
+
       {/* Right side actions */}
       <div
         className="ml-2 flex shrink-0 items-center gap-1"

src/renderer/hooks/useAutoScrollBottom.ts

@@ -138,6 +138,8 @@ export function useAutoScrollBottom(
   const disabledRef = useRef(disabled);
   // Track resetKey to detect changes
   const prevResetKeyRef = useRef(resetKey);
+  // Set true when resetKey changes; consumed by the content effect to force scroll on first load
+  const needsInitialScrollRef = useRef(false);
 
   /**
    * Check if the scroll container is at the bottom.
@@ -223,34 +225,47 @@ export function useAutoScrollBottom(
     disabledRef.current = disabled;
   }, [disabled]);
 
-  // Reset isAtBottom state when resetKey changes (e.g., tab/session switch)
+  // Reset isAtBottom state when resetKey changes (e.g., tab/session switch).
-  // This ensures new content will auto-scroll to bottom
+  // Sets needsInitialScrollRef so the content effect scrolls to bottom on first load.
   useEffect(() => {
     if (resetKey !== prevResetKeyRef.current) {
       isAtBottomRef.current = true;
       wasAtBottomBeforeUpdateRef.current = true;
       prevResetKeyRef.current = resetKey;
+      needsInitialScrollRef.current = true;
     }
   }, [resetKey]);
 
   /**
-   * After content updates (dependencies change), scroll to bottom if we were at bottom.
+   * After content updates (dependencies change), scroll to bottom if:
+   * - User was already near the bottom before the update, OR
+   * - This is the first load after a tab/session switch (needsInitialScrollRef)
+   * Uses double-RAF + cleanup so React StrictMode's double-invoke doesn't fire twice.
    */
   useEffect(() => {
     // Skip if disabled (e.g., during navigation) or not enabled
     if (!enabled || disabled) return;
 
-    // Use requestAnimationFrame to ensure DOM has updated
+    let id1 = 0;
-    requestAnimationFrame(() => {
+    let id2 = 0;
-      // Re-check disabled state inside RAF - it might have changed between effect and callback
-      // This prevents auto-scroll from firing if navigation started after the effect ran
-      if (disabledRef.current) return;
 
-      // Only auto-scroll if user was at bottom before the update
+    id1 = requestAnimationFrame(() => {
-      if (wasAtBottomBeforeUpdateRef.current) {
+      id2 = requestAnimationFrame(() => {
-        scrollToBottom(autoBehavior);
+        // Re-check disabled state — navigation may have started between effect and RAF
-      }
+        if (disabledRef.current) return;
+
+        const shouldScroll = needsInitialScrollRef.current || wasAtBottomBeforeUpdateRef.current;
+        if (shouldScroll) {
+          needsInitialScrollRef.current = false;
+          scrollToBottom(autoBehavior);
+        }
+      });
     });
+
+    return () => {
+      cancelAnimationFrame(id1);
+      cancelAnimationFrame(id2);
+    };
     // eslint-disable-next-line react-hooks/exhaustive-deps -- Dynamic dependencies array is intentional design
   }, [...dependencies, enabled, disabled, autoBehavior, scrollToBottom]);