fix(desktop): skip lone-tilde escaping on directive lines; align prose tests with the unknown-tag escape

apps/desktop/src/lib/markdown-preprocess.directives.test.ts

@@ -58,8 +58,11 @@ describe('preprocessMarkdown / directive lines', () => {
   })
 
   it('does not touch prose that merely contains ::', () => {
+    // The `::` itself is never structure, but `<int>` is an unknown html-like
+    // token: escaping it to entities is the #53953 behavior (the renderer
+    // would otherwise swallow it), and it paints identically as literal text.
     const text = 'Use std::vector<int> for *speed*.'
 
-    expect(preprocessMarkdown(text)).toBe(text)
+    expect(preprocessMarkdown(text)).toBe('Use std::vector&lt;int&gt; for *speed*.')
   })
 })

apps/desktop/src/lib/markdown-preprocess.reasoning.test.ts

@@ -19,6 +19,12 @@ describe('reasoning blocks in streamed markdown', () => {
     expect(preprocessMarkdown('Answer.\n<div')).toBe('Answer.\n<div')
 
     const quoted = 'O texto acima explica o formato do bloco <thinking> sem nunca fechá-lo'
-    expect(preprocessMarkdown(quoted)).toBe(quoted)
+    // The mid-sentence mention is escaped to entities by the #53953 unknown-tag
+    // pass — the renderer would otherwise read it as an unclosed tag — so the
+    // round-trip differs from the source by exactly that escape, and the
+    // escaped form paints identically as literal text.
+    expect(preprocessMarkdown(quoted)).toBe(
+      'O texto acima explica o formato do bloco &lt;thinking&gt; sem nunca fechá-lo'
+    )
   })
 })

apps/desktop/src/lib/markdown-preprocess.ts

@@ -102,6 +102,10 @@ const URL_LIKE_SPLIT_RE = /(<https?:\/\/[^>\s]+>|https?:\/\/[^\s<>"'`*]+[^\s<>"'
 // literal tilde, so CJK ranges (`1~10`) and approximation prefixes (`~¥0.089`)
 // no longer pair up into a GFM strikethrough span.
 const LONE_TILDE_RE = /(?<![\\~])~(?!~)/g
+// Same shape as DIRECTIVE_LINE_RE, anchored to a single line: escapeLoneTildes
+// tests one line at a time and must not carry the shared regex's `g` flag
+// (stateful lastIndex would skip every other directive line).
+const DIRECTIVE_LINE_ONLY_RE = /^[ \t]*::[a-z][a-z0-9-]{0,63}\{[^{}\n]{0,1024}\}[ \t]*$/
 // HTML-shaped prose tokens (`<tool_call>`, `<observation>`...) that are NOT
 // real inline elements get swallowed by the HTML-aware renderer (parse5 sees
 // an unclosed tag and consumes the rest of the message). Match unknown tag-like
@@ -313,7 +317,22 @@ function autoLinkRawUrls(text: string): string {
 function escapeLoneTildes(text: string): string {
   return text
     .split(URL_LIKE_SPLIT_RE)
-    .map(part => (/^<?https?:\/\//i.test(part) ? part : part.replace(LONE_TILDE_RE, '\\~')))
+    .map(part => {
+      if (/^<?https?:\/\//i.test(part)) {
+        return part
+      }
+
+      // Directive lines are shielded verbatim further down the pipeline:
+      // shieldDirectiveLines backslash-escapes every inline metachar (`~`
+      // included) so the card value arrives as ONE text node. Escaping a `~`
+      // here would leave `\\~` after the shield doubles the backslash, and the
+      // parser then emits the stray `\` into the directive's rendered value
+      // (#50871 follow-up: `brief="Sync ~/notes to ~/backup nightly"`).
+      return part
+        .split('\n')
+        .map(line => (DIRECTIVE_LINE_ONLY_RE.test(line) ? line : line.replace(LONE_TILDE_RE, '\\~')))
+        .join('\n')
+    })
     .join('')
 }