fix(ui): unify scrollbar styles across scrollable containers

Add .scrollbar-thin and .scrollbar-thin-edge utility classes in
globals.css and apply them to sidebar, file tree, git changes,
git log, session files, diff preview, and message thread panels.
Replace scattered inline webkit-scrollbar overrides with the
shared classes for consistent appearance and gutter behavior.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/app/globals.css

@@ -174,6 +174,22 @@
   }
 }
 
+/* Unified scrollbar style for scrollable containers.
+   Matches StickToBottom's internal scrollbar-gutter so all scroll areas
+   behave the same: symmetric gutter, no layout shift. */
+.scrollbar-thin {
+  scrollbar-width: thin;
+  scrollbar-color: var(--border) transparent;
+  scrollbar-gutter: stable both-edges;
+}
+
+/* Single-edge variant: only reserves space on the scrollbar side */
+.scrollbar-thin-edge {
+  scrollbar-width: thin;
+  scrollbar-color: var(--border) transparent;
+  scrollbar-gutter: stable;
+}
+
 /* Streamdown code blocks: dark mode via shiki dual-theme CSS variables */
 .dark [data-streamdown="code-block-body"] {
   background-color: var(--shiki-dark-bg, var(--sdm-bg, transparent)) !important;

src/components/conversations/sidebar-conversation-list.tsx

@@ -482,11 +482,8 @@ export function SidebarConversationList({
             <div
               ref={scrollContainerRef}
               className={cn(
-                "flex-1 min-h-0 overflow-y-auto px-1.5",
+                "flex-1 min-h-0 overflow-y-auto scrollbar-thin",
-                "[overflow-anchor:none]",
+                "[overflow-anchor:none]"
-                "[&::-webkit-scrollbar]:w-1.5",
-                "[&::-webkit-scrollbar-thumb]:rounded-full",
-                "[&::-webkit-scrollbar-thumb]:bg-border"
               )}
             >
               <Virtualizer ref={virtualizerRef} itemSize={CARD_HEIGHT}>

src/components/diff/unified-diff-preview.tsx

@@ -546,7 +546,7 @@ export function UnifiedDiffPreview({
 
   if (files.length === 0) {
     return (
-      <div className={cn("h-full overflow-auto", className)}>
+      <div className={cn("h-full overflow-auto scrollbar-thin", className)}>
         <pre className="font-mono text-[11px] leading-5 whitespace-pre-wrap text-muted-foreground p-3">
           {diffText}
         </pre>
@@ -555,7 +555,7 @@ export function UnifiedDiffPreview({
   }
 
   return (
-    <div className={cn("h-full overflow-auto", className)}>
+    <div className={cn("h-full overflow-auto scrollbar-thin", className)}>
       <div className="space-y-3">
         {files.map((file) => {
           const newFile = isNewFileOnly(file)
@@ -586,7 +586,7 @@ export function UnifiedDiffPreview({
                 )}
               </header>
 
-              <div className="overflow-auto">
+              <div className="overflow-auto scrollbar-thin">
                 <div className="inline-block min-w-full">
                   {newFile
                     ? file.hunks.map((hunk) => (

src/components/files/file-workspace-panel.tsx

@@ -693,7 +693,7 @@ function DiffFileList({
           </p>
         )}
       </div>
-      <div className="flex-1 min-h-0 overflow-y-auto">
+      <div className="flex-1 min-h-0 overflow-y-auto scrollbar-thin">
         <div className="py-1">
           {diffOutline.files.map((file) => (
             <ContextMenu key={file.key}>

src/components/layout/aux-panel-file-tree-tab.tsx

@@ -2167,7 +2167,7 @@ export function FileTreeTab() {
     <div className="flex flex-col h-full">
       <ContextMenu>
         <ContextMenuTrigger asChild>
-          <div className="flex-1 min-h-0 overflow-auto pb-1 [scrollbar-gutter:stable] [&::-webkit-scrollbar]:h-1.5 [&::-webkit-scrollbar]:w-1.5 [&::-webkit-scrollbar-thumb]:rounded-full [&::-webkit-scrollbar-thumb]:bg-border">
+          <div className="flex-1 min-h-0 overflow-auto pb-1 scrollbar-thin-edge">
             <FileTree
               key={folder?.path ?? "file-tree-empty"}
               className="border-0 rounded-none bg-transparent w-max min-w-full"

src/components/layout/aux-panel-git-changes-tab.tsx

@@ -1287,7 +1287,7 @@ export function GitChangesTab() {
 
   return (
     <>
-      <div className="h-full min-h-0 overflow-y-auto">
+      <div className="h-full min-h-0 overflow-y-auto scrollbar-thin-edge">
         {trackedChanges.length === 0 && untrackedChanges.length === 0 ? (
           <div className="flex items-center justify-center h-full p-4">
             <p className="text-xs text-muted-foreground text-center">

src/components/layout/aux-panel-git-log-tab.tsx

@@ -903,7 +903,7 @@ export function GitLogTab() {
 
   if (loading) {
     return (
-      <div className="flex flex-col h-full overflow-y-auto p-2">
+      <div className="flex flex-col h-full overflow-y-auto scrollbar-thin px-1 py-3">
         {hasBranches && (
           <BranchSelector
             branchList={branchList}
@@ -929,7 +929,7 @@ export function GitLogTab() {
 
   if (error) {
     return (
-      <div className="flex flex-col h-full overflow-y-auto p-2">
+      <div className="flex flex-col h-full overflow-y-auto scrollbar-thin px-1 py-3">
         {hasBranches && (
           <BranchSelector
             branchList={branchList}
@@ -959,7 +959,7 @@ export function GitLogTab() {
 
   if (entries.length === 0) {
     return (
-      <div className="flex flex-col h-full overflow-y-auto p-2">
+      <div className="flex flex-col h-full overflow-y-auto scrollbar-thin px-1 py-3">
         {hasBranches && (
           <BranchSelector
             branchList={branchList}
@@ -985,7 +985,7 @@ export function GitLogTab() {
         <ContextMenuTrigger asChild>
           <div
             onScroll={handleScroll}
-            className="flex-1 min-h-0 overflow-y-auto p-2 space-y-2"
+            className="flex-1 min-h-0 overflow-y-auto scrollbar-thin px-1 py-3 space-y-3"
           >
             {hasBranches && (
               <div

src/components/layout/aux-panel-session-files-tab.tsx

@@ -103,7 +103,7 @@ function SessionFilesContent({ conversationId }: { conversationId: number }) {
   }
 
   return (
-    <div className="space-y-3 p-3">
+    <div className="space-y-3 px-1 py-3">
       {groups.map((group, groupIndex) => {
         const groupKey = `${group.userTurnId}-${group.timestamp}-${groupIndex}`
         const isOpen = openGroups[groupKey] ?? false
@@ -290,7 +290,7 @@ export function SessionFilesTab() {
 
   return (
     <div className="flex flex-col h-full">
-      <div className="flex-1 min-h-0 overflow-y-auto">
+      <div className="flex-1 min-h-0 overflow-y-auto scrollbar-thin">
         <SessionFilesContent conversationId={conversationId} />
       </div>
     </div>

src/components/message/virtualized-message-thread.tsx

@@ -73,7 +73,7 @@ export function VirtualizedMessageThread<T>({
   return (
     <MessageThreadContent
       className={cn("mx-0 max-w-none p-0", contentClassName)}
-      scrollClassName="[overflow-anchor:none]"
+      scrollClassName="scrollbar-thin [overflow-anchor:none]"
       {...contentProps}
     >
       {items.length === 0 ? (
@@ -88,7 +88,7 @@ export function VirtualizedMessageThread<T>({
               key={getItemKey(item, index)}
               style={itemStyle(index, items.length)}
             >
-              <div className={cn("mx-auto max-w-3xl px-4", className)}>
+              <div className={cn("mx-auto max-w-3xl px-2", className)}>
                 {renderItem(item, index)}
               </div>
             </div>