diff --git a/__tests__/unit/node/markdown/lineMap.test.ts b/__tests__/unit/node/markdown/lineMap.test.ts
new file mode 100644
index 00000000..0ab4fa18
--- /dev/null
+++ b/__tests__/unit/node/markdown/lineMap.test.ts
@@ -0,0 +1,65 @@
+import {
+  LineMap,
+  MappedBuilder,
+  sliceSegments,
+  type LineMapSegment
+} from 'node/markdown/lineMap'
+
+describe('node/markdown/lineMap', () => {
+  test('resolve walks segments and extrapolates past the last one', () => {
+    const map = new LineMap([
+      { start: 0, file: 'a.md', line: 0 },
+      { start: 3, file: 'b.md', line: 10 },
+      { start: 5, file: 'a.md', line: 4 }
+    ])
+    expect(map.resolve(0)).toEqual({ file: 'a.md', line: 0 })
+    expect(map.resolve(2)).toEqual({ file: 'a.md', line: 2 })
+    expect(map.resolve(3)).toEqual({ file: 'b.md', line: 10 })
+    expect(map.resolve(4)).toEqual({ file: 'b.md', line: 11 })
+    expect(map.resolve(9)).toEqual({ file: 'a.md', line: 8 })
+  })
+
+  test('builder maps appended chunks and coalesces continuations', () => {
+    const b = new MappedBuilder()
+    b.append('one\ntwo\n', [{ start: 0, file: 'a.md', line: 0 }])
+    // continues exactly where a.md left off — no new segment needed
+    b.append('three\n', [{ start: 0, file: 'a.md', line: 2 }])
+    b.append('inc-1\ninc-2\n', [{ start: 0, file: 'b.md', line: 7 }])
+    const { src, segments } = b.build()
+    expect(src).toBe('one\ntwo\nthree\ninc-1\ninc-2\n')
+    expect(segments).toEqual([
+      { start: 0, file: 'a.md', line: 0 },
+      { start: 3, file: 'b.md', line: 7 }
+    ])
+  })
+
+  test('mid-line appends leave the line with its starter and mark it spliced', () => {
+    const b = new MappedBuilder()
+    b.append('before ', [{ start: 0, file: 'a.md', line: 4 }])
+    b.append('x\ny\n', [{ start: 0, file: 'b.md', line: 0 }])
+    b.append('tail\n', [{ start: 0, file: 'a.md', line: 5 }])
+    const { src, segments, splicedLines } = b.build()
+    expect(src).toBe('before x\ny\ntail\n')
+    expect(segments).toEqual([
+      { start: 0, file: 'a.md', line: 4 },
+      { start: 1, file: 'b.md', line: 1 },
+      { start: 2, file: 'a.md', line: 5 }
+    ])
+    expect([...splicedLines]).toEqual([0])
+  })
+
+  test('sliceSegments intersects and rebases', () => {
+    const segments: LineMapSegment[] = [
+      { start: 0, file: 'a.md', line: 0 },
+      { start: 4, file: 'b.md', line: 20 }
+    ]
+    expect(sliceSegments(segments, 2, 6)).toEqual([
+      { start: 0, file: 'a.md', line: 2 },
+      { start: 2, file: 'b.md', line: 20 }
+    ])
+    expect(sliceSegments(segments, 4, 5)).toEqual([
+      { start: 0, file: 'b.md', line: 20 }
+    ])
+    expect(sliceSegments(segments, 0, 0)).toEqual([])
+  })
+})
diff --git a/src/node/siteConfig.ts b/src/node/siteConfig.ts
index 418121ae..31381e20 100644
--- a/src/node/siteConfig.ts
+++ b/src/node/siteConfig.ts
@@ -79,9 +79,6 @@ export interface TransformPageContext<ThemeConfig = any> {
   siteConfig: SiteConfig<ThemeConfig>
 }
 
-/**
- * VitePress config, usually defined in `.vitepress/config.[ext]`.
- */
 /**
  * Where and how a checked link was authored, passed to `ignoreDeadLinks`
  * filter functions.
@@ -104,6 +101,9 @@ export interface DeadLinkContext {
   url: string
 }
 
+/**
+ * VitePress config, usually defined in `.vitepress/config.[ext]`.
+ */
 export interface UserConfig<
   ThemeConfig = any
 > extends LocaleSpecificConfig<ThemeConfig> {