Resizable left sidebar

Make left sidebar resizable so long URLs can be viewed in their
entirety.

Fixes https://github.com/modelcontextprotocol/inspector/issues/457

Vibe-coded with Cursor

client/src/App.tsx

@@ -28,7 +28,7 @@ import React, {
   useState,
 } from "react";
 import { useConnection } from "./lib/hooks/useConnection";
-import { useDraggablePane } from "./lib/hooks/useDraggablePane";
+import { useDraggablePane, useDraggableSidebar } from "./lib/hooks/useDraggablePane";
 import { StdErrNotification } from "./lib/notificationTypes";
 
 import { Tabs, TabsContent, TabsList, TabsTrigger } from "@/components/ui/tabs";
@@ -163,6 +163,7 @@ const App = () => {
   const progressTokenRef = useRef(0);
 
   const { height: historyPaneHeight, handleDragStart } = useDraggablePane(300);
+  const { width: sidebarWidth, isDragging: isSidebarDragging, handleDragStart: handleSidebarDragStart } = useDraggableSidebar(320);
 
   const {
     connectionStatus,
@@ -562,32 +563,44 @@ const App = () => {
 
   return (
     <div className="flex h-screen bg-background">
-      <Sidebar
-        connectionStatus={connectionStatus}
-        transportType={transportType}
-        setTransportType={setTransportType}
-        command={command}
-        setCommand={setCommand}
-        args={args}
-        setArgs={setArgs}
-        sseUrl={sseUrl}
-        setSseUrl={setSseUrl}
-        env={env}
-        setEnv={setEnv}
-        config={config}
-        setConfig={setConfig}
-        bearerToken={bearerToken}
-        setBearerToken={setBearerToken}
-        headerName={headerName}
-        setHeaderName={setHeaderName}
-        onConnect={connectMcpServer}
-        onDisconnect={disconnectMcpServer}
-        stdErrNotifications={stdErrNotifications}
-        logLevel={logLevel}
-        sendLogLevelRequest={sendLogLevelRequest}
-        loggingSupported={!!serverCapabilities?.logging || false}
-        clearStdErrNotifications={clearStdErrNotifications}
-      />
+      <div
+        style={{ width: sidebarWidth, minWidth: 200, maxWidth: 600, transition: isSidebarDragging ? 'none' : 'width 0.15s' }}
+        className="bg-card border-r border-border flex flex-col h-full relative"
+      >
+        <Sidebar
+          connectionStatus={connectionStatus}
+          transportType={transportType}
+          setTransportType={setTransportType}
+          command={command}
+          setCommand={setCommand}
+          args={args}
+          setArgs={setArgs}
+          sseUrl={sseUrl}
+          setSseUrl={setSseUrl}
+          env={env}
+          setEnv={setEnv}
+          config={config}
+          setConfig={setConfig}
+          bearerToken={bearerToken}
+          setBearerToken={setBearerToken}
+          headerName={headerName}
+          setHeaderName={setHeaderName}
+          onConnect={connectMcpServer}
+          onDisconnect={disconnectMcpServer}
+          stdErrNotifications={stdErrNotifications}
+          logLevel={logLevel}
+          sendLogLevelRequest={sendLogLevelRequest}
+          loggingSupported={!!serverCapabilities?.logging || false}
+          clearStdErrNotifications={clearStdErrNotifications}
+        />
+        {/* Drag handle for resizing sidebar */}
+        <div
+          onMouseDown={handleSidebarDragStart}
+          style={{ cursor: 'col-resize', position: 'absolute', top: 0, right: 0, width: 6, height: '100%', zIndex: 10, background: isSidebarDragging ? 'rgba(0,0,0,0.08)' : 'transparent' }}
+          aria-label="Resize sidebar"
+          data-testid="sidebar-drag-handle"
+        />
+      </div>
       <div className="flex-1 flex flex-col overflow-hidden">
         <div className="flex-1 overflow-auto">
           {mcpClient ? (

client/src/components/Sidebar.tsx

@@ -214,7 +214,7 @@ const Sidebar = ({
   }, [generateMCPServerFile, toast, reportError]);
 
   return (
-    <div className="w-80 bg-card border-r border-border flex flex-col h-full">
+    <div className="bg-card border-r border-border flex flex-col h-full">
       <div className="flex items-center justify-between p-4 border-b border-gray-200 dark:border-gray-800">
         <div className="flex items-center">
           <h1 className="ml-2 text-lg font-semibold">

client/src/lib/hooks/useDraggablePane.ts

@@ -51,3 +51,52 @@ export function useDraggablePane(initialHeight: number) {
     handleDragStart,
   };
 }
+
+export function useDraggableSidebar(initialWidth: number) {
+  const [width, setWidth] = useState(initialWidth);
+  const [isDragging, setIsDragging] = useState(false);
+  const dragStartX = useRef<number>(0);
+  const dragStartWidth = useRef<number>(0);
+
+  const handleDragStart = useCallback(
+    (e: React.MouseEvent) => {
+      setIsDragging(true);
+      dragStartX.current = e.clientX;
+      dragStartWidth.current = width;
+      document.body.style.userSelect = "none";
+    },
+    [width],
+  );
+
+  const handleDragMove = useCallback(
+    (e: MouseEvent) => {
+      if (!isDragging) return;
+      const deltaX = e.clientX - dragStartX.current;
+      const newWidth = Math.max(200, Math.min(600, dragStartWidth.current + deltaX));
+      setWidth(newWidth);
+    },
+    [isDragging],
+  );
+
+  const handleDragEnd = useCallback(() => {
+    setIsDragging(false);
+    document.body.style.userSelect = "";
+  }, []);
+
+  useEffect(() => {
+    if (isDragging) {
+      window.addEventListener("mousemove", handleDragMove);
+      window.addEventListener("mouseup", handleDragEnd);
+      return () => {
+        window.removeEventListener("mousemove", handleDragMove);
+        window.removeEventListener("mouseup", handleDragEnd);
+      };
+    }
+  }, [isDragging, handleDragMove, handleDragEnd]);
+
+  return {
+    width,
+    isDragging,
+    handleDragStart,
+  };
+}