fix(core): correct range intersection when ranges share start/end lines

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 },

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 },