diff --git a/CHANGELOG.md b/CHANGELOG.md
index 04cbdb5..6ac8768 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,10 @@
 # Hermes Web UI -- Changelog
 
+## [v0.50.102] — 2026-04-20
+
+### Fixed
+- **Code blocks no longer lose newlines when not preceded by a blank line** — `renderMd()` in `static/ui.js` now stashes `<pre>` blocks (including language-labelled blocks with `<div class="pre-header">` wrappers), mermaid diagrams, and katex blocks before the paragraph-splitting pass, then restores them. Previously, if a fenced code block was not separated from surrounding text by a double newline, all `\n` inside it were replaced with `<br>`, causing Prism.js to collapse the entire block to one line and misidentify everything after a `//` comment as a comment. (Fixes #745, reported by @qqxpee)
+
 ## [v0.50.101] — 2026-04-20
 
 ### Fixed
diff --git a/static/ui.js b/static/ui.js
index 08b7ad7..69c9434 100644
--- a/static/ui.js
+++ b/static/ui.js
@@ -646,8 +646,18 @@ function renderMd(raw){
     }
     return `<span class="katex-inline" data-katex="inline">${esc(item.src)}</span>`;
   });
+  // Stash rendered <pre> blocks (with optional pre-header div) and mermaid/katex
+  // divs before paragraph splitting so \n inside code blocks is never replaced
+  // with <br>. Token \x00E (next free after B D F G L M C O A).
+  // Fixes #745: code blocks collapse to single line when not preceded by blank line.
+  const _pre_stash=[];
+  s=s.replace(/(<div class="pre-header">[\s\S]*?<\/div>)?<pre>[\s\S]*?<\/pre>|<div class="(mermaid-block|katex-block)"[\s\S]*?<\/div>/g,m=>{
+    _pre_stash.push(m);
+    return '\x00E'+(_pre_stash.length-1)+'\x00';
+  });
   const parts=s.split(/\n{2,}/);
-  s=parts.map(p=>{p=p.trim();if(!p)return '';if(/^<(h[1-6]|ul|ol|pre|hr|blockquote)/.test(p))return p;return `<p>${p.replace(/\n/g,'<br>')}</p>`;}).join('\n');
+  s=parts.map(p=>{p=p.trim();if(!p)return '';if(/^<(h[1-6]|ul|ol|pre|hr|blockquote)|^\x00E/.test(p))return p;return `<p>${p.replace(/\n/g,'<br>')}</p>`;}).join('\n');
+  s=s.replace(/\x00E(\d+)\x00/g,(_,i)=>_pre_stash[+i]);
   // ── Restore MEDIA stash → inline images or download links ─────────────────
   s=s.replace(/\x00D(\d+)\x00/g,(_,i)=>{
     const ref=media_stash[+i];
diff --git a/tests/test_745_code_block_newlines.py b/tests/test_745_code_block_newlines.py
new file mode 100644
index 0000000..de3f716
--- /dev/null
+++ b/tests/test_745_code_block_newlines.py
@@ -0,0 +1,104 @@
+"""
+Tests for #745: code blocks losing newlines when not preceded by double blank line.
+
+Root cause: the paragraph-splitter in renderMd() replaced \n with <br> inside
+<pre><code> blocks when they were not separated by a double newline from surrounding
+text. The fix stashes <pre> blocks (and pre-header divs, mermaid, katex) before
+the paragraph split and restores them afterwards.
+"""
+import re
+import subprocess
+import sys
+import os
+
+UI_JS = os.path.join(os.path.dirname(__file__), '..', 'static', 'ui.js')
+
+
+def get_ui_js():
+    return open(UI_JS, encoding='utf-8').read()
+
+
+class TestCodeBlockNewlinePreservation:
+
+    def test_pre_stash_present(self):
+        """The _pre_stash variable must exist in ui.js."""
+        src = get_ui_js()
+        assert '_pre_stash' in src, "_pre_stash not found in ui.js"
+
+    def test_pre_stash_token_E_used(self):
+        """Stash token \\x00E must be used for pre-block stashing."""
+        src = get_ui_js()
+        assert r'\x00E' in src, r"\x00E stash token not found in ui.js"
+
+    def test_stash_before_paragraph_split(self):
+        """_pre_stash must be populated BEFORE the parts=s.split line."""
+        src = get_ui_js()
+        pre_stash_pos = src.index('_pre_stash=[]')
+        split_pos = src.index('const parts=s.split(/\\n{2,}/)')
+        assert pre_stash_pos < split_pos, \
+            "_pre_stash must be initialised before the paragraph split"
+
+    def test_restore_after_paragraph_split(self):
+        """_pre_stash restore must happen AFTER the paragraph map/join line."""
+        src = get_ui_js()
+        restore_pos = src.index('_pre_stash[+i]')
+        split_pos = src.index("}).join('\\n');", src.index('const parts=s.split'))
+        assert restore_pos > split_pos, \
+            "_pre_stash must be restored after the paragraph split/join"
+
+    def test_paragraph_split_bypasses_stash_tokens(self):
+        """The paragraph map must bypass lines that start with \\x00E."""
+        src = get_ui_js()
+        # The map line must check for \x00E in its bypass condition
+        map_line = next(
+            l for l in src.splitlines()
+            if 'parts.map' in l and '<br>' in l
+        )
+        assert r'\x00E' in map_line, \
+            r"paragraph map must bypass \x00E stash tokens"
+
+    def test_pre_regex_covers_pre_header_div(self):
+        """The stash regex must match <div class=\"pre-header\"> before <pre>."""
+        src = get_ui_js()
+        # Find the replacement regex used to populate _pre_stash
+        stash_block_idx = src.index('_pre_stash=[]')
+        stash_block = src[stash_block_idx:stash_block_idx + 400]
+        assert 'pre-header' in stash_block, \
+            "pre-stash regex must match <div class=\"pre-header\"> wrappers"
+
+    def test_mermaid_covered_by_stash(self):
+        """The stash regex must also cover mermaid-block divs."""
+        src = get_ui_js()
+        stash_block_idx = src.index('_pre_stash=[]')
+        stash_block = src[stash_block_idx:stash_block_idx + 400]
+        assert 'mermaid-block' in stash_block, \
+            "pre-stash regex must cover mermaid-block divs"
+
+    def test_katex_covered_by_stash(self):
+        """The stash regex must also cover katex-block divs."""
+        src = get_ui_js()
+        stash_block_idx = src.index('_pre_stash=[]')
+        stash_block = src[stash_block_idx:stash_block_idx + 400]
+        assert 'katex-block' in stash_block, \
+            "pre-stash regex must cover katex-block divs"
+
+    def test_js_syntax_valid(self):
+        """ui.js must pass node --check after the fix."""
+        result = subprocess.run(
+            ['node', '--check', UI_JS],
+            capture_output=True, text=True
+        )
+        assert result.returncode == 0, \
+            f"node --check failed:\n{result.stderr}"
+
+    def test_stash_token_e_not_used_elsewhere(self):
+        """\\x00E must only appear in the pre-stash section (not reused)."""
+        src = get_ui_js()
+        occurrences = [
+            i for i in range(len(src))
+            if src[i:i+4] == r'\x00' and i + 4 < len(src) and src[i+4] == 'E'
+        ]
+        # Allow 2 occurrences: the push token and the restore regex
+        # (may be 3 if there's also a comment mentioning it)
+        assert len(occurrences) >= 2, \
+            r"Expected at least 2 uses of \x00E (push + restore)"