Give the file name the row, and stop repeating the project

On a real project the name lost the width fight and the trailing text read
"MyGame.Runtime MyGame.Runtime" — the directory and the project were the
same string, printed twice.

- The detail text now has a shrink factor of 1000 against the name's 1, plus
  a 45% cap, so it gives up its width first and the name is the last thing to
  ellipsize.
- A directory that already begins with the project name says everything the
  project name would, so only one of the two is shown.
- verticalTabs.showDirectory became never / duplicates / always, defaulting
  to duplicates: width goes to a directory only when two open tabs share a
  file name, which is the usual reason to want it. Turning showProject off
  as well gives the name the whole row.
- The trailing text is composed in the extension now rather than the webview,
  so the deduplication is unit-testable and there is one implementation.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/test/suite/tabs.test.ts

@@ -4,7 +4,9 @@ import * as vscode from 'vscode';
 
 import { closeOthers, closeTab, togglePin } from '../../actions';
 import { projectColorIndex, projectOf } from '../../projects';
-import { TabEntry, buildEntries, describeEntry, sortEntries, tabUri } from '../../tabs';
+import {
+    TabEntry, buildEntries, describeEntry, duplicateLabels, sortEntries, tabUri,
+} from '../../tabs';
 
 /** Files in three different projects of the test solution. */
 const FILES = [
@@ -240,23 +242,53 @@ suite('Tab list', () => {
         assert.ok(settings.id.startsWith('1:label:'), `unexpected id ${settings.id}`);
     });
 
-    test('the description shows directory, project and only shows the group when split', () => {
+    test('the detail text never repeats the project inside the directory', () => {
+        const all = { showProject: true, showDirectory: true, showColumn: false };
+
+        // A .csproj-per-directory layout: the directory already names the project.
+        const same = { label: 'Profiler.cs', dir: 'Nerfed.Runtime', project: 'Nerfed.Runtime' } as TabEntry;
+        assert.strictEqual(describeEntry(same, all), 'Nerfed.Runtime');
+
+        // A subdirectory of the project: still no need to repeat it.
+        const nested = {
+            label: 'Transform.cs', dir: 'Nerfed.Runtime/Util', project: 'Nerfed.Runtime',
+        } as TabEntry;
+        assert.strictEqual(describeEntry(nested, all), 'Nerfed.Runtime/Util');
+
+        // A Unity-style layout, where the directory says nothing about the assembly.
+        const unrelated = {
+            label: 'ClientTranslator.cs', dir: 'Assets/Scripts/Client', project: 'Acme.Shop.Client',
+        } as TabEntry;
+        assert.strictEqual(describeEntry(unrelated, all),
+            'Assets/Scripts/Client  Acme.Shop.Client');
+    });
+
+    test('the detail text can be reduced to nothing', () => {
         const entry = {
             label: 'Profiler.cs', dir: 'Nerfed.Runtime', project: 'Nerfed.Runtime', column: 1,
         } as TabEntry;
 
-        assert.strictEqual(
-            describeEntry(entry, { showProject: true, showDirectory: true, showColumn: false }),
-            'Nerfed.Runtime  [Nerfed.Runtime]');
         assert.strictEqual(
             describeEntry(entry, { showProject: false, showDirectory: true, showColumn: false }),
             'Nerfed.Runtime');
+        assert.strictEqual(
+            describeEntry(entry, { showProject: true, showDirectory: false, showColumn: false }),
+            'Nerfed.Runtime');
         assert.strictEqual(
             describeEntry(entry, { showProject: false, showDirectory: false, showColumn: true }),
             '#1');
         assert.strictEqual(
             describeEntry(entry, { showProject: false, showDirectory: false, showColumn: false }),
-            '');
+            '', 'the file name should be able to have the whole row');
+    });
+
+    test('only names shared by more than one tab count as ambiguous', () => {
+        const entry = (label: string) => ({ label } as TabEntry);
+        assert.deepStrictEqual(
+            [...duplicateLabels([entry('a.cs'), entry('b.cs'), entry('a.cs')])],
+            ['a.cs']);
+        assert.deepStrictEqual([...duplicateLabels([entry('a.cs'), entry('b.cs')])], []);
+        assert.deepStrictEqual([...duplicateLabels([])], []);
     });
 
     test('sorting is stable for entries that compare equal', () => {