WIP: preserve failed Codex job issue-242-hhdxc839

Refs #454

## Automated implementation (requires human review)
[SidebarComponent.tsx](frontend/src/components/layout/SidebarComponent.tsx) の1行を修正しました。`overflow-y-auto` を `md` 以上に限定し、スマホでサゼストがサイドバーの領域外で切られる原因を取り除きました。

差分を確認済みです。指定に従い、テスト・lint・ビルドは未実行です。実画面でのスマホ表示とPC表示の確認も未実施です。

## Automated review (not a guarantee of correctness)
```text
The change restricts sidebar scrolling to medium and larger viewports. No actionable regression was identified in the current diff.

```

## Verification
- Tests/builds/migrations were not authorized and were not invoked by the controller.
- Codex was instructed not to run them; its command transcript should be inspected.
- Do not merge before manual acceptance and required checks.

---------

Co-authored-by: Codex Night Worker <codex-night@localhost>
Reviewed-on: #456
Co-authored-by: テツノニジカ <969+codex-bot@noreply.localhost>

backend/db/migrate/20261115000000_add_post_count_index_to_tags.rb

@@ -0,0 +1,5 @@
+class AddPostCountIndexToTags < ActiveRecord::Migration[8.0]
+  def change
+    add_index :tags, :post_count
+  end
+end

backend/db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.0].define(version: 2026_10_15_010000) do
+ActiveRecord::Schema[8.0].define(version: 2026_11_15_000000) do
   create_table "active_storage_attachments", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.string "name", null: false
     t.string "record_type", null: false
@@ -630,6 +630,7 @@ ActiveRecord::Schema[8.0].define(version: 2026_10_15_010000) do
     t.integer "post_count", default: 0, null: false
     t.integer "version_no", null: false
     t.index ["deprecated_at"], name: "index_tags_on_deprecated_at"
+    t.index ["post_count"], name: "index_tags_on_post_count"
     t.check_constraint "(`deprecated_at` is null) or (`category` <> _utf8mb4'nico')", name: "chk_tags_deprecated_at_not_nico"
     t.check_constraint "`version_no` > 0", name: "chk_tags_version_no_positive"
   end

backend/spec/models/tag_spec.rb

@@ -2,6 +2,19 @@ require 'rails_helper'
 
 RSpec.describe Tag, type: :model do
   include WikiRecords
+
+  describe 'post_count index' do
+    it 'has a non-unique index on post_count alone' do
+      indexes = described_class.connection.indexes(described_class.table_name)
+
+      expect(indexes).to include(
+          have_attributes(
+              name: 'index_tags_on_post_count',
+              columns: ['post_count'],
+              unique: false))
+    end
+  end
+
   describe '#has_description?' do
     let(:tag) { create(:tag) }
 

frontend/src/components/layout/SidebarComponent.test.tsx

@@ -70,6 +70,16 @@ describe ('SidebarComponent', () => {
     vi.spyOn (HTMLElement.prototype, 'hasPointerCapture').mockReturnValue (true)
   })
 
+  it ('limits sidebar scrolling to desktop to avoid clipping mobile suggestions', () => {
+    renderSidebar ()
+
+    // jsdom has no layout; guard the responsive classes responsible for clipping.
+    const sidebar = screen.getByText ('Sidebar body')
+
+    expect (sidebar).not.toHaveClass ('overflow-y-auto')
+    expect (sidebar).toHaveClass ('md:overflow-y-auto')
+  })
+
   it ('uses maxWidth without applying the container main-width reserve twice', async () => {
     localStorage.setItem ('sidebar:test-sidebar:left:width', '700')
     const onWidthChange = vi.fn ()

frontend/src/components/layout/SidebarComponent.tsx

@@ -339,7 +339,7 @@ const SidebarComponent: FC<Props> = ({
 	  </style>
 	</Helmet>
 
-	<div className="h-full overflow-y-auto p-4 sidebar">
+	<div className="h-full md:overflow-y-auto p-4 sidebar">
 	  {children}
 	</div>
 

frontend/src/pages/tags/TagDetailPage.test.tsx

@@ -35,6 +35,10 @@ const renderPage = () =>
 describe ('TagDetailPage', () => {
   beforeEach (() => {
     vi.clearAllMocks ()
+    tagsApi.fetchTag.mockReset ()
+    api.apiPut.mockReset ()
+    api.isApiError.mockReset ()
+    toastApi.toast.mockReset ()
     api.isApiError.mockReturnValue (false)
   })
 
@@ -51,8 +55,11 @@ describe ('TagDetailPage', () => {
   })
 
   it ('submits edited tag fields', async () => {
-    tagsApi.fetchTag.mockResolvedValueOnce (buildTag ({ id: 7, name: 'old' }))
-    api.apiPut.mockResolvedValueOnce (buildTag ({ id: 7, name: 'new', aliases: ['alias'] }))
+    const updatedTag = buildTag ({ id: 7, name: 'new', aliases: ['alias'] })
+    tagsApi.fetchTag
+	.mockResolvedValue (updatedTag)
+	.mockResolvedValueOnce (buildTag ({ id: 7, name: 'old' }))
+    api.apiPut.mockResolvedValueOnce (updatedTag)
 
     renderPage ()