fix(web): keep a lone dollar on a later line inside the block

packages/client/ui-primitives/src/markdown/mathCompatibility.ts

@@ -236,8 +236,10 @@ function createMathFlow(marker: number, openMarker: number, closeMarker: number)
       // that line's ending while a later `$$` on the same line still closes; on
       // any later line it ends the candidate outright, so a following block is
       // not swallowed. A solitary `$` is content, not a failed fence.
-      if (crossedLineEnding) return nok(code)
-      if (sawClosingFenceSequence) failedClose = true
+      if (sawClosingFenceSequence) {
+        if (crossedLineEnding) return nok(code)
+        failedClose = true
+      }
       return markerValueStart(code)
     }
 

packages/client/ui-primitives/tests/markdown.client.spec.tsx

@@ -656,6 +656,12 @@ describe('MarkdownText', () => {
     }
   })
 
+  it('keeps a lone dollar on a later line inside the block', () => {
+    const { container } = render(<MarkdownText text={'$$a+b\nc$d\n$$'} />)
+
+    expect(container.querySelector('.katex-error')?.textContent).toBe('a+b\nc$d')
+  })
+
   it('keeps a fence that fails inside a comment and a trailing lone dollar parseable', () => {
     const commentFail = render(<MarkdownText text={'$$x%$$ y'} />)
     expect(commentFail.container.querySelector('.katex-display')).toBeNull()