Fix dropped and duplicated lines for nested collapse ranges

`parseSections` in packages/@expressive-code/plugin-collapsible-sections/src/utils.ts skips any `collapse` range that overlaps a section it already collected, because the `<details>`-based rendering cannot handle overlapping sections. The check only tested whether the new range's `from` or `to` falls inside an existing section, so a range that fully contains an existing one, e.g. `collapse={3-5, 1-8}`, was not recognized as overlapping and got added anyway. Both sections then reached `sectionizeAst` in src/ast.ts, which splices sections into the line array from last to first while reading their content from the unmodified `lines` array. With `collapse={3-5, 1-8}` on a 10-line block, splicing the 1-8 section shrinks the array, so the following splice for 3-5 removes line 10 and appends copies of lines 3 to 5: rendered code that no longer matches the source. Replaces the two partial checks with the standard interval overlap test `from <= existingTo && existingFrom <= to`, which covers containment in both directions and keeps the previous results for every range combination that was already handled.
MaxFreedomPollard · Sep 7, 2026 · Sep 7, 2026 · 34ba418d3d05b0f12344f7d797df79a15b6de2e4
commit 34ba418d3d05b0f12344f7d797df79a15b6de2e4
diff --git a/.changeset/sour-lions-repeat.md b/.changeset/sour-lions-repeat.md
@@ -0,0 +1,5 @@
+---
+'@expressive-code/plugin-collapsible-sections': patch
+---
+
+Fixes code lines being dropped and duplicated when a `collapse` range fully contains a range that was listed before it, e.g. `collapse={3-5, 1-8}`. Ranges like this are now skipped, just like any other range that overlaps an already added section.
diff --git a/packages/@expressive-code/plugin-collapsible-sections/src/utils.ts b/packages/@expressive-code/plugin-collapsible-sections/src/utils.ts
@@ -20,9 +20,9 @@ export function parseSections(value: string): Section[] {
 			if (from > to) return
 
 			// skip any entries that overlap existing sections, since our <details>-based approach can't overlap
+			// (this includes entries that fully contain an existing section, e.g. '3-5, 1-8')
 			for (const { from: existingFrom, to: existingTo } of sections) {
-				if (from >= existingFrom && from <= existingTo) return
-				if (to >= existingFrom && to <= existingTo) return
+				if (from <= existingTo && existingFrom <= to) return
 			}
 
 			sections.push({ from, to, lines: [] })

diff --git a/packages/@expressive-code/plugin-collapsible-sections/test/preprocess-meta.test.ts b/packages/@expressive-code/plugin-collapsible-sections/test/preprocess-meta.test.ts
@@ -136,4 +136,14 @@ test('Merges overlapping sections', async () => {
 	await expectMetaResult('collapse={2-5,1-2}', {
 		sections: [{ from: 2, to: 5 }],
 	})
+
+	// A range that fully contains an already added section overlaps it, too,
+	// so it must be skipped no matter which of the two ranges comes first
+	await expectMetaResult('collapse={3-5,1-8}', {
+		sections: [{ from: 3, to: 5 }],
+	})
+
+	await expectMetaResult('collapse={1-8,3-5}', {
+		sections: [{ from: 1, to: 8 }],
+	})
 })
diff --git a/packages/@expressive-code/plugin-collapsible-sections/test/rendering.test.ts b/packages/@expressive-code/plugin-collapsible-sections/test/rendering.test.ts
@@ -1,4 +1,5 @@
 import { describe, test, expect } from 'vitest'
+import type { Element } from '@expressive-code/core/hast'
 import { getClassNames, select, selectAll, toText } from '@expressive-code/core/hast'
 import { pluginShiki } from '@expressive-code/plugin-shiki'
 import { pluginTextMarkers } from '@expressive-code/plugin-text-markers'
@@ -97,6 +98,24 @@ describe('Renders collapsed sections', async () => {
 			})
 		})
 
+		test(`Ignores sections that fully contain an already added section`, async ({ task: { name: testName } }) => {
+			const validateSections = buildBlockValidationFn([{ from: 5, to: 7, text: '3 collapsed lines' }])
+			await renderAndOutputHtmlSnapshot({
+				testName,
+				testBaseDir: __dirname,
+				fixtures: buildThemeFixtures(themes, {
+					code: lineMarkerTestText,
+					meta: `collapse={5-7, 1-9}`,
+					plugins: [pluginCollapsibleSections()],
+					blockValidationFn: (context) => {
+						validateSections(context)
+						// Also ensure that no code lines were dropped or duplicated
+						expect(getRenderedCodeLines(context.renderedGroupAst)).toEqual(lineMarkerTestText.split('\n'))
+					},
+				}),
+			})
+		})
+
 		test(`Correctly handles code with text-markers and syntax highlighting`, { timeout: 5 * 1000 }, async ({ task: { name: testName } }) => {
 			await renderAndOutputHtmlSnapshot({
 				testName,
@@ -177,6 +196,18 @@ describe('Renders collapsed sections', async () => {
 	})
 })
 
+/** Returns the text of all rendered code lines, including the lines hidden inside collapsed sections */
+function getRenderedCodeLines(renderedGroupAst: Element) {
+	const codeAst = select('pre > code', renderedGroupAst)
+	if (!codeAst) throw new Error("Couldn't find code AST when collecting rendered code lines")
+
+	// Replace each section with the lines it contains (= all of its children except the summary)
+	const lines = codeAst.children.flatMap((child) => ('tagName' in child && child.tagName.toLowerCase() === 'details' ? child.children.slice(1) : [child]))
+
+	// Empty code lines are rendered as a single line break, so strip it to get the source text back
+	return lines.map((line) => toText(line, { whitespace: 'pre' }).replace(/\n$/, ''))
+}
+
 type ExpectedSection = Omit<Section, 'lines'> & {
 	text: string
 }