Don’t create list item for valid horizontal rules

Valid horizontal rules can look like a list item, such as
`- —`.  We should not create a new list item for these

app/assets/javascripts/lib/utils/text_markdown.js

@@ -11,6 +11,9 @@ const LINK_TAG_PATTERN = '[{text}](url)';
 // followed by one or more whitespace characters
 const LIST_LINE_HEAD_PATTERN = /^(?<indent>\s*)(?<leader>((?<isUl>[*+-])|(?<isOl>\d+\.))( \[([xX ])\])?\s)(?<content>.)?/;
 
+// detect a horizontal rule that might be mistaken for a list item (not full pattern for an <hr>)
+const HR_PATTERN = /^((\s{0,3}-+\s*-+\s*-+\s*[\s-]*)|(\s{0,3}\*+\s*\*+\s*\*+\s*[\s*]*))$/;
+
 function selectedText(text, textarea) {
   return text.substring(textarea.selectionStart, textarea.selectionEnd);
 }
@@ -381,13 +384,15 @@ function handleContinueList(e, textArea) {
 
     let itemToInsert;
 
+    // Behaviors specific to either `ol` or `ul`
     if (isOl) {
       const nextLine = lineAfter(textArea.value, textArea, false);
       const nextLineResult = nextLine.match(LIST_LINE_HEAD_PATTERN);
 
       itemToInsert = continueOlText(result, nextLineResult);
     } else {
-      // isUl
+      if (currentLine.match(HR_PATTERN)) return;
+
       itemToInsert = `${indent}${leader}`;
     }
 

spec/frontend/lib/utils/text_markdown_spec.js

@@ -178,10 +178,18 @@ describe('init markdown', () => {
         it.each`
           text                           | expected
           ${'- item'}                    | ${'- item\n- '}
+          ${'* item'}                    | ${'* item\n* '}
+          ${'+ item'}                    | ${'+ item\n+ '}
           ${'- [ ] item'}                | ${'- [ ] item\n- [ ] '}
           ${'- [x] item'}                | ${'- [x] item\n- [ ] '}
           ${'- [X] item'}                | ${'- [X] item\n- [ ] '}
           ${'- item\n  - second'}        | ${'- item\n  - second\n  - '}
+          ${'- - -'}                     | ${'- - -'}
+          ${'- --'}                      | ${'- --'}
+          ${'* **'}                      | ${'* **'}
+          ${' **  * ** * ** * **'}       | ${' **  * ** * ** * **'}
+          ${'- - -x'}                    | ${'- - -x\n- '}
+          ${'+ ++'}                      | ${'+ ++\n+ '}
           ${'1. item'}                   | ${'1. item\n2. '}
           ${'1. [ ] item'}               | ${'1. [ ] item\n2. [ ] '}
           ${'1. [x] item'}               | ${'1. [x] item\n2. [ ] '}