From f7bb09b654e8d41899b71d58b869d9a1af0632ee Mon Sep 17 00:00:00 2001
From: eeshsaxena <139802361+eeshsaxena@users.noreply.github.com>
Date: Sat, 29 Aug 2026 10:03:57 +0530
Subject: [PATCH] fix(core): correct range intersection when ranges share
 start/end lines

In the multi-line branch, intersection picked the start character from
whichever range's start line matched startLine first, and likewise for the end.
When both ranges begin (or end) on the shared line, that dropped the
comparison: it returned a's character instead of the later start / earlier end.
Compare the characters when the lines are equal.
---
 core/util/ranges.test.ts | 18 ++++++++++++++++++
 core/util/ranges.ts      | 15 +++++++++++++--
 2 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/core/util/ranges.test.ts b/core/util/ranges.test.ts
index 84d1c8d54a2..9466c003f61 100644
--- a/core/util/ranges.test.ts
+++ b/core/util/ranges.test.ts
@@ -353,6 +353,24 @@ describe("intersection", () => {
     });
   });
 
+  test("uses later start and earlier end when both ranges share the start and end lines", () => {
+    rangeA = {
+      start: { line: 0, character: 5 },
+      end: { line: 3, character: 4 },
+    };
+
+    rangeB = {
+      start: { line: 0, character: 10 },
+      end: { line: 3, character: 2 },
+    };
+
+    const result = intersection(rangeA, rangeB);
+    expect(result).toEqual({
+      start: { line: 0, character: 10 },
+      end: { line: 3, character: 2 },
+    });
+  });
+
   test("returns correct intersection when ranges touch at the edge", () => {
     rangeA = {
       start: { line: 1, character: 0 },
diff --git a/core/util/ranges.ts b/core/util/ranges.ts
index a9bf5e97242..978fea58c2c 100644
--- a/core/util/ranges.ts
+++ b/core/util/ranges.ts
@@ -46,10 +46,21 @@ export function intersection(a: Range, b: Range): Range | null {
     };
   }
 
+  // When both ranges begin on the shared start line, the intersection begins at
+  // the later of the two characters (and symmetrically ends at the earlier one).
+  // Picking whichever range's line matched first dropped that comparison.
   const startCharacter =
-    startLine === a.start.line ? a.start.character : b.start.character;
+    a.start.line === b.start.line
+      ? Math.max(a.start.character, b.start.character)
+      : startLine === a.start.line
+        ? a.start.character
+        : b.start.character;
   const endCharacter =
-    endLine === a.end.line ? a.end.character : b.end.character;
+    a.end.line === b.end.line
+      ? Math.min(a.end.character, b.end.character)
+      : endLine === a.end.line
+        ? a.end.character
+        : b.end.character;
 
   return {
     start: { line: startLine, character: startCharacter },