From fb827422a63239da04b34b5842e9e44993d8032b Mon Sep 17 00:00:00 2001
From: Raunak Raj <71929976+bajrangCoder@users.noreply.github.com>
Date: Sun, 26 Jul 2026 22:19:22 +0530
Subject: [PATCH] feat(editor): add indentation folding for legacy modes(as
 fallback)

---
 src/cm/baseExtensions.ts     |  3 +-
 src/cm/indentationFolding.ts | 69 ++++++++++++++++++++++++++++++++++++
 src/test/editor.tests.js     | 30 ++++++++++++++++
 3 files changed, 101 insertions(+), 1 deletion(-)
 create mode 100644 src/cm/indentationFolding.ts

diff --git a/src/cm/baseExtensions.ts b/src/cm/baseExtensions.ts
index 927c3ed943..336027f51a 100644
--- a/src/cm/baseExtensions.ts
+++ b/src/cm/baseExtensions.ts
@@ -15,6 +15,7 @@ import {
 import { highlightSelectionMatches } from "@codemirror/search";
 import type { Extension } from "@codemirror/state";
 import { EditorState, Prec } from "@codemirror/state";
+import indentationFolding from "./indentationFolding";
 import {
 	crosshairCursor,
 	drawSelection,
@@ -56,7 +57,7 @@ export default function createBaseExtensions(
 	];
 
 	if (enableHighlightActiveLine) extensions.push(highlightActiveLineGutter());
-	if (codeFolding) extensions.push(foldGutter());
+	if (codeFolding) extensions.push(foldGutter(), indentationFolding);
 	extensions.push(drawSelection());
 	extensions.push(dropCursor());
 	extensions.push(EditorState.allowMultipleSelections.of(true));
diff --git a/src/cm/indentationFolding.ts b/src/cm/indentationFolding.ts
new file mode 100644
index 0000000000..7be802cbfa
--- /dev/null
+++ b/src/cm/indentationFolding.ts
@@ -0,0 +1,69 @@
+import {
+	foldService,
+	language,
+	StreamLanguage,
+} from "@codemirror/language";
+import { countColumn, type EditorState } from "@codemirror/state";
+
+interface FoldRange {
+	from: number;
+	to: number;
+}
+
+const foldCache = new WeakMap<
+	EditorState,
+	Map<number, FoldRange | null>
+>();
+
+function findIndentationFold(
+	state: EditorState,
+	from: number,
+): FoldRange | null {
+	const activeLanguage = state.facet(language);
+	if (activeLanguage && !(activeLanguage instanceof StreamLanguage)) {
+		return null;
+	}
+
+	const line = state.doc.lineAt(from);
+	let stateCache = foldCache.get(state);
+	if (!stateCache) {
+		stateCache = new Map();
+		foldCache.set(state, stateCache);
+	}
+	if (stateCache.has(line.from)) return stateCache.get(line.from) ?? null;
+
+	const indentationEnd = line.text.search(/\S|$/);
+	if (indentationEnd === line.text.length) {
+		stateCache.set(line.from, null);
+		return null;
+	}
+
+	const indentation = countColumn(line.text, state.tabSize, indentationEnd);
+	let foldEnd: number | null = null;
+
+	for (
+		let lineNumber = line.number + 1;
+		lineNumber <= state.doc.lines;
+		lineNumber++
+	) {
+		const nextLine = state.doc.line(lineNumber);
+		const nextIndentationEnd = nextLine.text.search(/\S|$/);
+		if (nextIndentationEnd === nextLine.text.length) continue;
+
+		const nextIndentation = countColumn(
+			nextLine.text,
+			state.tabSize,
+			nextIndentationEnd,
+		);
+		if (nextIndentation <= indentation) break;
+
+		foldEnd = nextLine.to;
+	}
+
+	const range =
+		foldEnd === null ? null : { from: line.to, to: foldEnd };
+	stateCache.set(line.from, range);
+	return range;
+}
+
+export default foldService.of(findIndentationFold);
diff --git a/src/test/editor.tests.js b/src/test/editor.tests.js
index 551311ab32..e9a6928494 100644
--- a/src/test/editor.tests.js
+++ b/src/test/editor.tests.js
@@ -10,9 +10,11 @@ import { javascript } from "@codemirror/lang-javascript";
 import {
 	bracketMatching,
 	defaultHighlightStyle,
+	foldable,
 	foldEffect,
 	foldedRanges,
 	foldGutter,
+	StreamLanguage,
 	syntaxHighlighting,
 } from "@codemirror/language";
 import { highlightSelectionMatches, searchKeymap } from "@codemirror/search";
@@ -754,6 +756,34 @@ export async function runCodeMirrorTests(writeOutput) {
 		},
 	);
 
+	runner.test(
+		"Legacy modes fold indentation while preserving blank lines",
+		(test) => {
+			const streamLanguage = StreamLanguage.define({
+				startState: () => ({}),
+				copyState: (state) => ({ ...state }),
+				token: (stream) => {
+					stream.skipToEnd();
+					return null;
+				},
+			});
+			const state = EditorState.create({
+				doc: "root\n\tchild\n\n\tchild2\nnext",
+				extensions: [
+					...createBaseExtensions(),
+					EditorState.tabSize.of(4),
+					streamLanguage,
+				],
+			});
+			const firstLine = state.doc.line(1);
+			const range = foldable(state, firstLine.from, firstLine.to);
+
+			test.assert(range != null, "Indented lines should be foldable");
+			test.assertEqual(range.from, firstLine.to);
+			test.assertEqual(range.to, state.doc.line(4).to);
+		},
+	);
+
 	runner.test(
 		"Fold all includes same-line nested blocks and unfold all clears them",
 		async (test) => {