diff --git a/src/Microsoft.SourceIndexer.Tasks/GenerateVmrRepoTags.cs b/src/Microsoft.SourceIndexer.Tasks/GenerateVmrRepoTags.cs
new file mode 100644
index 0000000..067291a
--- /dev/null
+++ b/src/Microsoft.SourceIndexer.Tasks/GenerateVmrRepoTags.cs
@@ -0,0 +1,100 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Text.Json;
+using Microsoft.Build.Framework;
+using Microsoft.Build.Utilities;
+
+namespace Microsoft.SourceIndexer.Tasks
+{
+    // Reads the dotnet/dotnet VMR's src/source-manifest.json and emits one repo tag per mapped
+    // subfolder, so search can be scoped to the originating repo (dotnet/runtime, dotnet/winforms, ...)
+    // instead of the whole VMR. Source links are unaffected -- this only feeds HtmlGenerator's
+    // /repoPath tagging, not /serverPath, so files still link back to dotnet/dotnet.
+    public class GenerateVmrRepoTags : Task
+    {
+        // Full path to the extracted VMR's src/source-manifest.json.
+        [Required]
+        public string ManifestPath { get; set; }
+
+        // Folder the manifest's repository paths are relative to (the extracted VMR's src directory).
+        [Required]
+        public string SrcRoot { get; set; }
+
+        // One item per repository: ItemSpec is the subfolder's full path, RepoDisplayName its owner/repo tag.
+        [Output]
+        public ITaskItem[] RepoTags { get; set; }
+
+        public override bool Execute()
+        {
+            try
+            {
+                ExecuteCore();
+                return !Log.HasLoggedErrors;
+            }
+            catch (Exception ex)
+            {
+                Log.LogErrorFromException(ex, true);
+                return false;
+            }
+        }
+
+        private void ExecuteCore()
+        {
+            RepoTags = Array.Empty<ITaskItem>();
+
+            if (!File.Exists(ManifestPath))
+            {
+                // Non-VMR builds (or a manifest that moved) simply produce no sub-tags.
+                Log.LogMessage(MessageImportance.Normal, $"Source manifest '{ManifestPath}' not found; no sub-repo tags generated.");
+                return;
+            }
+
+            using var doc = JsonDocument.Parse(File.ReadAllText(ManifestPath));
+            if (!doc.RootElement.TryGetProperty("repositories", out var repositories) ||
+                repositories.ValueKind != JsonValueKind.Array)
+            {
+                Log.LogWarning($"Source manifest '{ManifestPath}' has no 'repositories' array; no sub-repo tags generated.");
+                return;
+            }
+
+            var tags = new List<ITaskItem>();
+            foreach (var repo in repositories.EnumerateArray())
+            {
+                var path = repo.TryGetProperty("path", out var p) ? p.GetString() : null;
+                var remoteUri = repo.TryGetProperty("remoteUri", out var r) ? r.GetString() : null;
+                if (string.IsNullOrEmpty(path) || string.IsNullOrEmpty(remoteUri))
+                {
+                    continue;
+                }
+
+                var item = new TaskItem(Path.GetFullPath(Path.Combine(SrcRoot, path)));
+                item.SetMetadata("RepoDisplayName", GetDisplayName(remoteUri));
+                tags.Add(item);
+            }
+
+            RepoTags = tags.ToArray();
+            Log.LogMessage(MessageImportance.Normal, $"Generated {tags.Count} sub-repo tag(s) from '{ManifestPath}'.");
+        }
+
+        // Turns a clone URL into an owner/repo tag, matching the RepoDisplayName convention used for
+        // top-level repositories (the URL minus the github.com prefix).
+        private static string GetDisplayName(string remoteUri)
+        {
+            var s = remoteUri.Trim();
+            if (s.EndsWith(".git", StringComparison.OrdinalIgnoreCase))
+            {
+                s = s.Substring(0, s.Length - ".git".Length);
+            }
+
+            const string prefix = "https://github.com/";
+            if (s.StartsWith(prefix, StringComparison.OrdinalIgnoreCase))
+            {
+                return s.Substring(prefix.Length).Trim('/');
+            }
+
+            // Fall back to the URL path for any non-github remote.
+            return new Uri(s).AbsolutePath.Trim('/');
+        }
+    }
+}
diff --git a/src/Microsoft.SourceIndexer.Tasks/Microsoft.SourceIndexer.Tasks.csproj b/src/Microsoft.SourceIndexer.Tasks/Microsoft.SourceIndexer.Tasks.csproj
index 3a69713..cb4c01e 100644
--- a/src/Microsoft.SourceIndexer.Tasks/Microsoft.SourceIndexer.Tasks.csproj
+++ b/src/Microsoft.SourceIndexer.Tasks/Microsoft.SourceIndexer.Tasks.csproj
@@ -8,6 +8,7 @@
     <PackageReference Include="Microsoft.Build.Framework" />
     <PackageReference Include="Microsoft.Build.Tasks.Core" />
     <PackageReference Include="Microsoft.Build.Utilities.Core" />
+    <PackageReference Include="System.Text.Json" />
     <PackageReference Include="SharpZipLib" />
   </ItemGroup>
 </Project>
diff --git a/src/SourceBrowser/TestCode/TestSolution/ThisIsAnIntentionallyEnormousFileNameDesignedToOverflowTheNavigationPaneHorizontallySoTheResizableSplitterAndStickyRepoFilterCanBeExercised.cs b/src/SourceBrowser/TestCode/TestSolution/ThisIsAnIntentionallyEnormousFileNameDesignedToOverflowTheNavigationPaneHorizontallySoTheResizableSplitterAndStickyRepoFilterCanBeExercised.cs
new file mode 100644
index 0000000..fd6a6ed
--- /dev/null
+++ b/src/SourceBrowser/TestCode/TestSolution/ThisIsAnIntentionallyEnormousFileNameDesignedToOverflowTheNavigationPaneHorizontallySoTheResizableSplitterAndStickyRepoFilterCanBeExercised.cs
@@ -0,0 +1,16 @@
+namespace TestSolution
+{
+    /// <summary>
+    /// Deliberately long file name and type name used to exercise the nav-pane layout: a
+    /// long result line must scroll within its own group (and a long tree node within the
+    /// Solution Explorer) without pushing the repo filter dropdown off-screen, and the
+    /// side-by-side panes stay resizable via the splitter. Search for "ThisIsAn" to see it.
+    /// </summary>
+    public static class ThisIsAnIntentionallyEnormousTypeNameDesignedToOverflowTheNavigationPaneHorizontallySoTheResizableSplitterAndStickyRepoFilterCanBeExercised
+    {
+        public static string ThisIsAnIntentionallyEnormousMethodNameThatAlsoContributesToTheHorizontalWidthOfASingleResultLineForTestingPurposes()
+        {
+            return "overflow";
+        }
+    }
+}
diff --git a/src/SourceBrowser/src/HtmlGenerator/Pass2-Finalization/SolutionFinalizer.SolutionExplorer.cs b/src/SourceBrowser/src/HtmlGenerator/Pass2-Finalization/SolutionFinalizer.SolutionExplorer.cs
index 26579de..c3c94df 100644
--- a/src/SourceBrowser/src/HtmlGenerator/Pass2-Finalization/SolutionFinalizer.SolutionExplorer.cs
+++ b/src/SourceBrowser/src/HtmlGenerator/Pass2-Finalization/SolutionFinalizer.SolutionExplorer.cs
@@ -1,5 +1,7 @@
 using System;
 using System.IO;
+using System.Net;
+using System.Text;
 using Microsoft.SourceBrowser.Common;
 using Folder = Microsoft.SourceBrowser.HtmlGenerator.Folder<Microsoft.SourceBrowser.HtmlGenerator.ProjectSkeleton>;
 
@@ -7,7 +9,7 @@ namespace Microsoft.SourceBrowser.HtmlGenerator
 {
     public partial class SolutionFinalizer
     {
-        private void WriteSolutionExplorer(Folder root = null)
+        private void WriteSolutionExplorer(bool emitAssemblyList, Folder root = null)
         {
             if (root == null)
             {
@@ -21,7 +23,7 @@ private void WriteSolutionExplorer(Folder root = null)
                 Log.Write("Solution Explorer...");
                 Markup.WriteSolutionExplorerPrefix(writer);
                 WriteFolder(root, writer);
-                Markup.WriteSolutionExplorerSuffix(writer);
+                Markup.WriteSolutionExplorerSuffix(writer, emitAssemblyList);
             }
         }
 
@@ -88,9 +90,84 @@ private void WriteFolder(Folder folder, StreamWriter writer)
         private void WriteProject(string assemblyName, string repoName, StreamWriter writer)
         {
             var projectExplorerText = GetProjectExplorerText(assemblyName, repoName);
-            if (!string.IsNullOrEmpty(projectExplorerText))
+            if (string.IsNullOrEmpty(projectExplorerText))
             {
-                writer.WriteLine(projectExplorerText);
+                return;
+            }
+
+            // Split the project's title row from its file subtree and write the subtree to a
+            // per-project fragment the client fetches on first expand (see expandCollapseFolder in
+            // scripts.js). The Solution Explorer document then carries only the repo/solution/project
+            // skeleton, so a large index no longer forces the browser to parse -- and build collapsed
+            // DOM for -- every project's whole file tree up front. Projects with no subtree, or any
+            // that don't split cleanly, fall back to being written inline exactly as before.
+            if (TrySplitProjectSubtree(projectExplorerText, out var titleHtml, out var folderOpenTag, out var subtreeInner)
+                && TryWriteProjectSubtreeFragment(assemblyName, subtreeInner, out var fragmentSrc))
+            {
+                writer.Write(titleHtml);
+                writer.Write(folderOpenTag.Insert(folderOpenTag.Length - 1, " data-src=\"" + WebUtility.HtmlEncode(fragmentSrc) + "\""));
+                writer.WriteLine("</div>");
+                return;
+            }
+
+            writer.WriteLine(projectExplorerText);
+        }
+
+        // GetProjectExplorerText returns a project title div immediately followed by its file-tree
+        // folder div: <div class="projectCSInSolution" ...>Name</div><div class="folder" ...>INNER</div>.
+        // Peel those apart so the title (and its data-repo hook) can stay inline while INNER is deferred.
+        private static bool TrySplitProjectSubtree(string html, out string titleHtml, out string folderOpenTag, out string subtreeInner)
+        {
+            titleHtml = folderOpenTag = subtreeInner = null;
+
+            const string closeTag = "</div>";
+            var titleEnd = html.IndexOf(closeTag, StringComparison.Ordinal);
+            if (titleEnd < 0)
+            {
+                return false;
+            }
+
+            titleEnd += closeTag.Length;
+            var folderOpen = html.IndexOf("<div", titleEnd, StringComparison.Ordinal);
+            if (folderOpen < 0)
+            {
+                return false;
+            }
+
+            var folderOpenEnd = html.IndexOf('>', folderOpen);
+            var lastClose = html.LastIndexOf(closeTag, StringComparison.Ordinal);
+            if (folderOpenEnd < 0 || lastClose <= folderOpenEnd)
+            {
+                return false;
+            }
+
+            var inner = html.Substring(folderOpenEnd + 1, lastClose - (folderOpenEnd + 1));
+            if (inner.Trim().Length == 0)
+            {
+                return false;
+            }
+
+            titleHtml = html.Substring(0, titleEnd);
+            folderOpenTag = html.Substring(folderOpen, folderOpenEnd - folderOpen + 1);
+            subtreeInner = inner;
+            return true;
+        }
+
+        private bool TryWriteProjectSubtreeFragment(string assemblyName, string subtreeInner, out string fragmentSrc)
+        {
+            fragmentSrc = assemblyName + "/" + Constants.SolutionExplorerFragment;
+            try
+            {
+                File.WriteAllText(
+                    Path.Combine(SolutionDestinationFolder, assemblyName, Constants.SolutionExplorerFragment),
+                    subtreeInner,
+                    Encoding.UTF8);
+                return true;
+            }
+            catch (Exception ex)
+            {
+                Log.Exception(ex, "Failed writing Solution Explorer fragment for " + assemblyName);
+                return false;
             }
         }
 
diff --git a/src/SourceBrowser/src/HtmlGenerator/Pass2-Finalization/SolutionFinalizer.cs b/src/SourceBrowser/src/HtmlGenerator/Pass2-Finalization/SolutionFinalizer.cs
index 2c67a3c..144f698 100644
--- a/src/SourceBrowser/src/HtmlGenerator/Pass2-Finalization/SolutionFinalizer.cs
+++ b/src/SourceBrowser/src/HtmlGenerator/Pass2-Finalization/SolutionFinalizer.cs
@@ -110,7 +110,7 @@ public void FinalizeProjects(
             IReadOnlyDictionary<string, IReadOnlyList<string>> configsByAssembly = null)
         {
             SortProcessedAssemblies();
-            WriteSolutionExplorer(solutionExplorerRoot);
+            WriteSolutionExplorer(emitAssemblyList, solutionExplorerRoot);
             CreateReferencesFiles(additionalReferencedSymbolIdsByAssembly, mergedDivergentReferencesByAssembly, configsByAssembly);
             CreateMasterDeclarationsIndex();
             CreateProjectMap();
diff --git a/src/SourceBrowser/src/HtmlGenerator/Program.cs b/src/SourceBrowser/src/HtmlGenerator/Program.cs
index 6e36e4a..145a67a 100644
--- a/src/SourceBrowser/src/HtmlGenerator/Program.cs
+++ b/src/SourceBrowser/src/HtmlGenerator/Program.cs
@@ -133,7 +133,7 @@ await IndexSolutionsAsync(options.Projects, options.Properties, federation, opti
                     // existed. Finalization happens directly, exactly as today -- this is what makes the
                     // no-config case trivially byte-identical (the code path is literally untouched).
                     FinalizeProjects(options.EmitAssemblyList, federation);
-                    WebsiteFinalizer.Finalize(runOutputRoot, options.EmitAssemblyList, federation, options.ShowBranding);
+                    WebsiteFinalizer.Finalize(runOutputRoot, federation, options.ShowBranding);
                 }
                 else
                 {
@@ -239,7 +239,7 @@ private static void RunConfigMergeIfNeeded(string outRoot, bool emitAssemblyList
                     // as the default/no-config path would, straight into the shared "index/".
                     Paths.SolutionDestinationFolder = Path.Combine(outRoot, "obj", configs[0]);
                     FinalizeProjects(emitAssemblyList, federation);
-                    WebsiteFinalizer.Finalize(outRoot, emitAssemblyList, federation, showBranding);
+                    WebsiteFinalizer.Finalize(outRoot, federation, showBranding);
                     return;
                 }
 
@@ -256,7 +256,7 @@ private static void RunConfigMergeIfNeeded(string outRoot, bool emitAssemblyList
                     StringComparer.Ordinal);
 
                 ConfigAwareProjectFinalizer.Finalize(configObjRoots, Paths.WebsiteDestinationFolder, emitAssemblyList, federation, axisTagsByConfig);
-                WebsiteFinalizer.Finalize(outRoot, emitAssemblyList, federation, showBranding);
+                WebsiteFinalizer.Finalize(outRoot, federation, showBranding);
             });
         }
 
@@ -595,7 +595,7 @@ private static void GenerateLooseFilesProject(string projectName, string solutio
 
     internal static class WebsiteFinalizer
     {
-        public static void Finalize(string destinationFolder, bool emitAssemblyList, Federation federation, bool showBranding)
+        public static void Finalize(string destinationFolder, Federation federation, bool showBranding)
         {
             string sourcePath = Assembly.GetEntryAssembly().Location;
             sourcePath = Path.GetDirectoryName(sourcePath);
@@ -611,7 +611,7 @@ public static void Finalize(string destinationFolder, bool emitAssemblyList, Fed
 
             StampOverviewHtmlWithDate(destinationFolder);
 
-            ApplyScriptsJsCustomizations(destinationFolder, emitAssemblyList, federation, showBranding);
+            ApplyScriptsJsCustomizations(destinationFolder, federation, showBranding);
         }
 
         private static void StampOverviewHtmlWithDate(string destinationFolder)
@@ -702,12 +702,12 @@ private static string GetSourceBrowserVersion()
         // The generated site can run either through SourceIndexServer (which serves wwwroot/scripts.js
         // as its baseline, byte-identical to the checked-in template) or as pure static files, where
         // the copy under index/ -- and, at runtime, SourceIndexServer's own RootPath handler, which is
-        // registered ahead of its wwwroot handler -- is what's actually served. All three toggles below
+        // registered ahead of its wwwroot handler -- is what's actually served. Both toggles below
         // used to independently re-read wwwroot/scripts.js and overwrite index/scripts.js, which meant
-        // combining more than one (e.g. /assemblylist with a federation, or either alongside
-        // /showBranding) silently discarded whichever ran first. They're composed into one read-modify
-        // sequence here so any combination of flags ends up in the final file.
-        private static void ApplyScriptsJsCustomizations(string destinationFolder, bool emitAssemblyList, Federation federation, bool showBranding)
+        // combining them (a federation alongside /showBranding) silently discarded whichever ran first.
+        // They're composed into one read-modify sequence here so any combination of flags ends up in
+        // the final file.
+        private static void ApplyScriptsJsCustomizations(string destinationFolder, Federation federation, bool showBranding)
         {
             var source = Path.Combine(destinationFolder, "wwwroot/scripts.js");
             if (!File.Exists(source))
@@ -718,12 +718,6 @@ private static void ApplyScriptsJsCustomizations(string destinationFolder, bool
             var text = File.ReadAllText(source);
             var changed = false;
 
-            if (emitAssemblyList)
-            {
-                text = text.Replace("/*USE_SOLUTION_EXPLORER*/true/*USE_SOLUTION_EXPLORER*/", "false");
-                changed = true;
-            }
-
             var sb = new StringBuilder();
             foreach (var server in federation.GetServers())
             {
diff --git a/src/SourceBrowser/src/HtmlGenerator/Utilities/Constants.cs b/src/SourceBrowser/src/HtmlGenerator/Utilities/Constants.cs
index 0d7d8e4..9c22a42 100644
--- a/src/SourceBrowser/src/HtmlGenerator/Utilities/Constants.cs
+++ b/src/SourceBrowser/src/HtmlGenerator/Utilities/Constants.cs
@@ -45,6 +45,9 @@ public class Constants
         public static readonly string ClassificationPunctuation = "punctuation";
         public static readonly string ProjectExplorer = "ProjectExplorer";
         public static readonly string SolutionExplorer = "SolutionExplorer";
+        // Per-project file subtree the merged Solution Explorer defers and the client fetches on
+        // first expand (see SolutionFinalizer.WriteProject), written alongside ProjectExplorer.html.
+        public static readonly string SolutionExplorerFragment = "SolutionExplorerFragment.html";
         public static readonly string HuffmanFileName = "Huffman.txt";
         public static readonly string TopReferencedAssemblies = "TopReferencedAssemblies";
         public static readonly string BaseMembersFileName = "BaseMembers";
diff --git a/src/SourceBrowser/src/HtmlGenerator/Utilities/Markup.cs b/src/SourceBrowser/src/HtmlGenerator/Utilities/Markup.cs
index 6d5105e..efdc11e 100644
--- a/src/SourceBrowser/src/HtmlGenerator/Utilities/Markup.cs
+++ b/src/SourceBrowser/src/HtmlGenerator/Utilities/Markup.cs
@@ -291,9 +291,15 @@ Solution Explorer
 <div id=""rootFolder"" style=""display: none;"" class=""folderTitle"">");
         }
 
-        public static void WriteSolutionExplorerSuffix(TextWriter writer)
+        public static void WriteSolutionExplorerSuffix(TextWriter writer, bool emitAssemblyList)
         {
-            writer.WriteLine("</div><script>onSolutionExplorerLoad();</script></body></html>");
+            // Reciprocal of the "solution explorer" link on the assembly list (results.html); only
+            // emitted when that list was generated, so it never points at an empty results page.
+            var assemblyListLink = emitAssemblyList
+                ? @"<div class=""note"">Try also browsing the <a href=""results.html"" class=""blueLink"">assembly list</a>.</div>"
+                : null;
+
+            writer.WriteLine("</div>" + assemblyListLink + "<script>onSolutionExplorerLoad();</script></body></html>");
         }
 
         public static void WriteNamespaceExplorerPrefix(string assemblyName, StreamWriter sw, string pathPrefix = "")
@@ -353,11 +359,11 @@ public static string GetResultsHtmlPrefix()
 <link rel=""stylesheet"" href=""styles.css"" />
 <script src=""scripts.js""></script>
 </head>
-<body onload=""onResultsLoad();"">
+<body class=""resultsBody"" onload=""onResultsLoad();"">
 <select id=""repo-filter"" style=""display:none"" aria-label=""Filter search to a repo""></select>
 <div id=""symbols"" aria-live=""polite"">
 <div class=""note"">
-Enter a type or member name or <a href=""/#q=assembly%20"" target=""_top"" class=""blueLink"" onclick=""populateSearchBox('assembly '); return false;"">filter the assembly list</a>.
+Enter a type or member name.
 </div>
 <div class=""resultGroup"">
 ";
diff --git a/src/SourceBrowser/src/SourceIndexServer/wwwroot/index.html b/src/SourceBrowser/src/SourceIndexServer/wwwroot/index.html
index 6760d27..f4f7526 100644
--- a/src/SourceBrowser/src/SourceIndexServer/wwwroot/index.html
+++ b/src/SourceBrowser/src/SourceIndexServer/wwwroot/index.html
@@ -21,6 +21,11 @@
                 class="navFrame"
                 src="results.html"
                 title="search-results"></iframe>
+        <div class="paneResizer"
+             id="paneResizer"
+             role="separator"
+             aria-orientation="vertical"
+             title="Drag to resize"></div>
         <iframe name="s"
                 id="s"
                 class="contentFrame"
diff --git a/src/SourceBrowser/src/SourceIndexServer/wwwroot/scripts.js b/src/SourceBrowser/src/SourceIndexServer/wwwroot/scripts.js
index 24aec48..b94bfac 100644
--- a/src/SourceBrowser/src/SourceIndexServer/wwwroot/scripts.js
+++ b/src/SourceBrowser/src/SourceIndexServer/wwwroot/scripts.js
@@ -182,6 +182,8 @@ function onPageLoaded() {
 
     top.name = "topFrame";
 
+    initPaneResizer();
+
     var query = document.location.search;
     if (query && query.slice(0, 3) == "?q=") {
         redirectLocation(top, "/#" + query.slice(1));
@@ -203,6 +205,82 @@ function onPageLoaded() {
     processHash();
 }
 
+// Makes the divider between the side-by-side nav and content panes draggable (desktop layout
+// only). The nav pane width is persisted on `top` and in localStorage so it survives frame
+// reloads and return visits. The mobile single-pane layout has no splitter (it's display:none)
+// and must not inherit a fixed pixel width, so the width is cleared whenever that layout is
+// active and reapplied on return to the side-by-side layout.
+function initPaneResizer() {
+    var resizer = document.getElementById("paneResizer");
+    var navFrame = document.getElementById("n");
+    var container = document.getElementById("splitter");
+    if (!resizer || !navFrame || !container) {
+        return;
+    }
+
+    var MIN_WIDTH = 200;
+    var mobile = window.matchMedia(
+        "(max-width: 700px), (orientation: landscape) and (max-height: 500px) and (pointer: coarse)");
+
+    function storedWidth() {
+        var value = parseInt(localStorage.getItem("navPaneWidth"), 10);
+        return isNaN(value) ? 0 : value;
+    }
+
+    function clamp(width) {
+        var max = container.clientWidth - MIN_WIDTH;
+        if (max < MIN_WIDTH) {
+            max = MIN_WIDTH;
+        }
+        return Math.min(Math.max(width, MIN_WIDTH), max);
+    }
+
+    // In the mobile layout the panes stack full-width; a leftover pixel width would override
+    // that, so drop it there and restore the saved width when back to side-by-side.
+    function applyLayout() {
+        if (mobile.matches) {
+            navFrame.style.width = "";
+        } else if (storedWidth()) {
+            navFrame.style.width = clamp(storedWidth()) + "px";
+        }
+    }
+
+    resizer.addEventListener("pointerdown", function (e) {
+        if (mobile.matches) {
+            return;
+        }
+
+        e.preventDefault();
+        document.body.classList.add("resizingPanes");
+
+        // Track the move/up on the document rather than the 6px resizer: once the drag starts the
+        // cursor leaves the handle, and the .resizingPanes rule disables the iframes' pointer
+        // events so these still reach us here.
+        function onMove(ev) {
+            var width = clamp(ev.clientX - container.getBoundingClientRect().left);
+            navFrame.style.width = width + "px";
+        }
+
+        function onUp() {
+            document.body.classList.remove("resizingPanes");
+            document.removeEventListener("pointermove", onMove);
+            document.removeEventListener("pointerup", onUp);
+            localStorage.setItem("navPaneWidth", parseInt(navFrame.style.width, 10));
+        }
+
+        document.addEventListener("pointermove", onMove);
+        document.addEventListener("pointerup", onUp);
+    });
+
+    if (mobile.addEventListener) {
+        mobile.addEventListener("change", applyLayout);
+    } else if (mobile.addListener) {
+        mobile.addListener(applyLayout);
+    }
+
+    applyLayout();
+}
+
 function onHeaderLoad() {
     ensureSearchBox();
     searchTimerID = -1;
@@ -251,6 +329,9 @@ function onResultsLoad() {
     ensureSearchBox();
     initRepoFilter(function () { runSearch(); });
 
+    // The pinned-note height (sticky-header offset) depends on the pane width via wrapping.
+    window.addEventListener("resize", function () { updateGroupHeaderOffset(); });
+
     if (searchBox && searchBox.value && searchBox.value.length > 2) {
         runSearch();
     }
@@ -803,6 +884,20 @@ function embedRepoFilterInNote(scope) {
 
     note.classList.add("hasFilter");
     note.appendChild(repoFilterElement);
+
+    updateGroupHeaderOffset(note);
+}
+
+// The top "N results found" note is pinned (position: sticky, top: 0). Sticky group headers must
+// sit just below it, so publish the note's rendered height as --groupHeaderTop for the CSS to use.
+// The height varies with the embedded filter dropdown and with wrapping, so recompute on resize.
+function updateGroupHeaderOffset(note) {
+    if (!note) {
+        note = document.querySelector("#symbols > .note");
+    }
+
+    var height = note ? note.getBoundingClientRect().height : 0;
+    document.documentElement.style.setProperty("--groupHeaderTop", Math.round(height) + "px");
 }
 
 // Hides/shows each project's subtree in the merged Solution Explorer (SolutionExplorer.html)
@@ -920,6 +1015,12 @@ function loadSearchResults(data) {
                 top.n.embedRepoFilterInNote(container);
             }
 
+            // Groups just got replaced, so re-measure the pinned note for the sticky-header offset
+            // (embedRepoFilterInNote only does this when a filter dropdown is present).
+            if (typeof top.n.updateGroupHeaderOffset === "function") {
+                top.n.updateGroupHeaderOffset();
+            }
+
             if (searchBox && searchBox.value && searchBox.value.length > 2) {
                 setHash("q=" + encodeURIComponent(searchBox.value));
             }
@@ -1578,6 +1679,36 @@ function expandCollapseFolder(capturedFolder, capturedPlusMinus, capturedFolderI
                 capturedFolderImage.src = pathToIcons + openFolderIcon;
             }
 
+            // A project whose file subtree was deferred (see SolutionFinalizer.WriteProject) carries a
+            // data-src instead of inline children. Fetch and inject it on first expand, then wire the
+            // injected subtree the same way makeFoldersCollapsible does the initial document.
+            var deferredSrc = capturedFolder.getAttribute ? capturedFolder.getAttribute("data-src") : null;
+            if (deferredSrc && !capturedFolder.everExpanded) {
+                capturedFolder.everExpanded = true;
+                var initFn = capturedFolder.initialize;
+                capturedFolder.initialize = null;
+                fetch(deferredSrc)
+                    .then(function (r) { return r.ok ? r.text() : ""; })
+                    .then(function (html) {
+                        capturedFolder.innerHTML = html;
+                        var nested = capturedFolder.querySelectorAll(".folder");
+                        for (var j = 0; j < nested.length; j++) {
+                            nested[j].style.display = "none";
+                            nested[j].initialize = initFn;
+                        }
+                        if (initFn) {
+                            initFn(capturedFolder);
+                        }
+                        for (var k = 0; k < capturedFolder.children.length; k++) {
+                            if (capturedFolder.children[k].className === 'folder') {
+                                addImagesToFolder(capturedFolder.children[k], folderIcon, openFolderIcon, pathToIcons);
+                            }
+                        }
+                    });
+                capturedFolder.style.display = 'block';
+                return;
+            }
+
             if (capturedFolder.initialize) {
                 capturedFolder.initialize(capturedFolder);
                 capturedFolder.initialize = null;
diff --git a/src/SourceBrowser/src/SourceIndexServer/wwwroot/styles.css b/src/SourceBrowser/src/SourceIndexServer/wwwroot/styles.css
index ae881c0..b011357 100644
--- a/src/SourceBrowser/src/SourceIndexServer/wwwroot/styles.css
+++ b/src/SourceBrowser/src/SourceIndexServer/wwwroot/styles.css
@@ -73,9 +73,9 @@ body {
 .navFrame {
     flex: none;
     width: 504px;
+    min-width: 200px;
     height: 100%;
     border: 0;
-    border-right: 1px solid var(--color-pane-border);
 }
 
 .contentFrame {
@@ -85,6 +85,29 @@ body {
     border: 0;
 }
 
+/* Draggable divider between the side-by-side panes (desktop only -- hidden in
+   the single-pane mobile layout below). Doubles as the visual border the
+   navFrame used to own. initPaneResizer() in scripts.js sets navFrame's width
+   from a drag here. */
+.paneResizer {
+    flex: none;
+    width: 6px;
+    height: 100%;
+    background: var(--color-pane-border);
+    cursor: col-resize;
+}
+
+.paneResizer:hover,
+body.resizingPanes .paneResizer {
+    background: var(--color-selection);
+}
+
+/* While dragging, the cursor passes over the iframes; disabling their pointer
+   events keeps mousemove flowing to the top document driving the resize. */
+body.resizingPanes iframe {
+    pointer-events: none;
+}
+
 /* On narrow screens show one pane at a time instead of a 504px nav pane
    crowding the content. The header toggle button (and the search / result-click
    flows in scripts.js) flip the body's showNav class to pick which pane is
@@ -114,6 +137,11 @@ body {
         display: none;
     }
 
+    /* The splitter only makes sense in the side-by-side layout. */
+    .paneResizer {
+        display: none;
+    }
+
     body.showNav .navFrame {
         display: block;
     }
@@ -859,6 +887,99 @@ a i {
     background-size: 10px 6px;
 }
 
+/* Keep the note strips -- the repo filter dropdown, "N results found:", "Generated in X ms",
+   and the "Try also browsing" footer link -- pinned and visible, and scroll only the result
+   list/tree between them. Without this a single long result line widens the page and pushes the
+   dropdown off-screen, forcing a horizontal scroll just to reach the filter. Applies to the two
+   nav-frame pages that carry the filter: results.html and SolutionExplorer.html. */
+body.resultsBody,
+body.solutionExplorerBody {
+    height: 100vh;
+    display: flex;
+    flex-direction: column;
+    overflow: hidden;
+}
+
+/* results.html: #symbols holds the (sticky) note strips and the scrolling result groups. The
+   footer "solution explorer" link is a sibling below it and stays put on its own. */
+body.resultsBody #symbols {
+    flex: 1 1 auto;
+    min-height: 0;
+    overflow-y: auto;
+    overflow-x: hidden;
+}
+
+body.resultsBody #symbols > .note:first-child {
+    position: sticky;
+    top: 0;
+    z-index: 2;
+}
+
+body.resultsBody #symbols > .note:last-child {
+    position: sticky;
+    bottom: 0;
+    z-index: 2;
+}
+
+body.resultsBody > .note {
+    flex: none;
+}
+
+/* Each result group spans the full pane width (no side margins) so its header tint reads as a
+   full-width band, and the group's own vertical rhythm is preserved by the bottom margin only. */
+body.resultsBody .resultGroup {
+    margin-left: 0;
+    margin-right: 0;
+}
+
+/* A long result line scrolls within the group's collapsible body -- not the whole group -- so the
+   header above it (which is a sibling of this scroller) stays full-width and can stick on scroll
+   instead of sliding sideways with the content. The small indent keeps items inset under the icon. */
+body.resultsBody .resultGroup > div {
+    overflow-x: auto;
+    padding-left: 8px;
+}
+
+/* Project/assembly group headers float at the top of the scroll area while their items scroll past,
+   so the group a result belongs to stays labeled. They stick just below the pinned "N results"
+   note; --groupHeaderTop is that note's measured height (set in embedRepoFilterInNote). */
+body.resultsBody .resultGroupHeader {
+    position: sticky;
+    top: var(--groupHeaderTop, 0px);
+    z-index: 1;
+}
+
+/* SolutionExplorer.html: the header note and footer link stay put; the tree scrolls between. */
+body.solutionExplorerBody > .note {
+    flex: none;
+}
+
+body.solutionExplorerBody #rootFolder {
+    flex: 1 1 auto;
+    min-height: 0;
+    overflow: auto;
+}
+
+/* Repo (and, on multi-solution sites, solution) grouping headers stay visible while the tree is
+   scrolled. Sticky top floats the header down as its subtree scrolls past so the current repo stays
+   labeled; sticky left plus a full-pane min-width keeps the tint spanning the view even after a long
+   path has scrolled the tree sideways -- otherwise the band would end at the pane's original right
+   edge. The opaque background (repoTitle already tints; solutionTitle gets one) stops scrolled rows
+   from showing through the pinned header. */
+body.solutionExplorerBody .repoTitle,
+body.solutionExplorerBody .solutionTitle {
+    position: sticky;
+    top: 0;
+    left: 0;
+    z-index: 1;
+    min-width: 100%;
+    width: max-content;
+    box-sizing: border-box;
+}
+
+body.solutionExplorerBody .solutionTitle {
+    background-color: #ffffff;
+}
 /* Hides a Solution Explorer project subtree that doesn't match the selected repo filter (see
    filterSolutionExplorerByRepo in scripts.js). A class rather than inline display is used so this
    doesn't fight with the folder expand/collapse logic, which also toggles display on the same
diff --git a/src/index/index.proj b/src/index/index.proj
index 875c079..079d932 100644
--- a/src/index/index.proj
+++ b/src/index/index.proj
@@ -3,6 +3,7 @@
   <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.props))\dir.props" />
 
   <UsingTask TaskName="DownloadStage1Index" AssemblyFile="$(SourceIndexerTasksAssembly)"/>
+  <UsingTask TaskName="GenerateVmrRepoTags" AssemblyFile="$(SourceIndexerTasksAssembly)"/>
 
   <PropertyGroup>
     <WhatIf Condition="'$(WhatIf)' != 'true'">false</WhatIf>
@@ -177,13 +178,26 @@
     </ItemGroup>
   </Target>
 
+  <!-- For VMR entries carrying a source manifest, emit one repo filter tag per subfolder so
+       search can be scoped to the originating repo. Only /repoPath (filter) tags are added;
+       source links keep pointing at the top-level repo. -->
+  <Target Name="GenerateSubRepoTags" DependsOnTargets="ResolveHashV2"
+          Outputs="%(ClonedRepositoryV2.Identity)">
+    <GenerateVmrRepoTags
+        Condition="'%(ClonedRepositoryV2.SubRepoManifest)' != ''"
+        ManifestPath="$([MSBuild]::NormalizePath('%(ClonedRepositoryV2.ExtractPath)', '%(ClonedRepositoryV2.SubRepoManifest)'))"
+        SrcRoot="$([MSBuild]::NormalizePath('%(ClonedRepositoryV2.ExtractPath)', 'src'))">
+      <Output TaskParameter="RepoTags" ItemName="SubRepoTag"/>
+    </GenerateVmrRepoTags>
+  </Target>
+
   <Target Name="BuildGenerator" Condition="!Exists('$(HtmlGeneratorExePath)')">
     <MSBuild Projects="$(SourcesDir)SourceBrowser/src/HtmlGenerator/HtmlGenerator.csproj" Targets="Restore;Build" Properties="SolutionDir=$(SourcesDir)SourceBrowser">
       <Output TaskParameter="TargetOutputs" PropertyName="HtmlGeneratorExePath"/>
     </MSBuild>
   </Target>
 
-  <Target Name="BuildIndex" DependsOnTargets="BuildGenerator;FindBinlogs;FindSolutions">
+  <Target Name="BuildIndex" DependsOnTargets="BuildGenerator;FindBinlogs;FindSolutions;GenerateSubRepoTags">
     <Error Condition="!Exists('$(HtmlGeneratorExePath)')" Text="Html generator executable not found."/>
     <RemoveDuplicates Inputs="@(BinlogToIndex)">
       <Output TaskParameter="Filtered" ItemName="_FilteredBinlog"/>
@@ -204,6 +218,7 @@
       <SourceIndexCmd>$(SourceIndexCmd) /in:"$(OutDir)index.list"</SourceIndexCmd>
       <SourceIndexCmd>$(SourceIndexCmd)@(ClonedRepository -> ' /repo:"%(LocalPath)"="%(RepoDisplayName)"="%(ServerPath)"', '')</SourceIndexCmd>
       <SourceIndexCmd>$(SourceIndexCmd)@(ClonedRepositoryV2 -> ' /repo:"%(LocalPath)"="%(RepoDisplayName)"="%(ServerPath)"', '')</SourceIndexCmd>
+      <SourceIndexCmd>$(SourceIndexCmd)@(SubRepoTag -> ' /repoPath:"%(Identity)"="%(RepoDisplayName)"', '')</SourceIndexCmd>
     </PropertyGroup>
     <Exec Command="$(SourceIndexCmd)" LogStandardErrorAsError="false"/>
     <Copy SourceFiles="@(OverwriteFiles)" DestinationFiles="@(OverwriteFiles -> '$(OutDir)index/%(RecursiveDir)%(Filename)%(Extension)')"/>
diff --git a/src/index/repositories.props b/src/index/repositories.props
index a5f5050..5213f01 100644
--- a/src/index/repositories.props
+++ b/src/index/repositories.props
@@ -22,6 +22,10 @@
     <RepositoryV2 Include="dotnet">
       <RepoName>dotnet-dotnet</RepoName>
       <Url>https://github.com/dotnet/dotnet</Url>
+      <!-- Emit a per-subfolder repo filter tag for each entry in the VMR's source manifest,
+           so search can be scoped to the originating repo (dotnet/runtime, ...). Source links
+           are unaffected: files still point at dotnet/dotnet. -->
+      <SubRepoManifest>src/source-manifest.json</SubRepoManifest>
     </RepositoryV2>
 
     <!-- Repos not in the VMR that still upload their own stage 1 data -->