fix: keep formatting out of unselected lines

resources/js/lib/markdown-format.ts

@@ -72,8 +72,10 @@ export function formatMarkdown(
     }
 
     const prefix = PREFIXES[format] ?? (() => '');
-    const from = value.lastIndexOf('\n', start - 1) + 1;
-    const lineEnd = value.indexOf('\n', end);
+    const from = start === 0 ? 0 : value.lastIndexOf('\n', start - 1) + 1;
+    // A selection ending just after a newline does not include the next line.
+    const lastSelected = end > start && value[end - 1] === '\n' ? end - 1 : end;
+    const lineEnd = value.indexOf('\n', lastSelected);
     const to = lineEnd === -1 ? value.length : lineEnd;
     const insert = value
         .slice(from, to)

tests/Unit/MarkdownFormat.test.ts

@@ -35,3 +35,21 @@ await test('prefixes every line the selection touches', () => {
     assert.deepEqual(apply('What shipped', 3, 3, 'heading'), ['## What shipped', '']);
     assert.deepEqual(apply('a\nquote', 2, 2, 'quote'), ['a\n> quote', '']);
 });
+
+await test('does not prefix the next line when selection ends after a newline', () => {
+    assert.deepEqual(apply('first\nsecond', 0, 6, 'bulleted'), [
+        '- first\nsecond',
+        '',
+    ]);
+    assert.deepEqual(apply('first\nsecond\nthird', 0, 13, 'numbered'), [
+        '1. first\n2. second\nthird',
+        '',
+    ]);
+});
+
+void test('formats a selected first-line newline without an invalid range', () => {
+    assert.deepEqual(apply('\nsecond', 0, 1, 'bulleted'), [
+        '- \nsecond',
+        '',
+    ]);
+});