diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2bf82c4c..b0516d95 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,12 @@
 
 ## [Unreleased]
 
+## [v0.51.580] — 2026-06-22 — Release UM (wrap markdown source previews)
+
+### Fixed
+
+- **Markdown source previews now wrap normal prose instead of forcing horizontal scroll.** Fenced code blocks tagged `md`, `markdown`, or `mdx` (the inline viewer with the small `MD` header and Copy button) now render in a dedicated wrapping source-preview style, so ordinary markdown text flows to the message width while true code/diff blocks keep their existing horizontal-scroll behavior. Thanks @TomBanksAU. (#4669)
+
 ## [v0.51.579] — 2026-06-22 — Release UL (deflake TLS end-to-end tests)
 
 ### Fixed
diff --git a/static/style.css b/static/style.css
index b81ce66e..4ee51c2b 100644
--- a/static/style.css
+++ b/static/style.css
@@ -1687,6 +1687,9 @@
   .msg-body code{font-family:"SF Mono","Fira Code",ui-monospace,monospace;font-size:var(--message-code-font-size);background:var(--code-inline-bg);padding:1px 5px;border-radius:4px;color:var(--code-text);}
   .msg-body pre{background:var(--code-bg);border:1px solid var(--border);border-radius:10px;padding:14px 16px;overflow-x:auto;margin:10px 0;}
   .msg-body pre code{background:none;padding:0;border-radius:0;color:var(--pre-text);font-size:var(--message-pre-code-font-size);line-height:1.6;}
+  .msg-body pre.md-source-block,.preview-md pre.md-source-block{white-space:pre-wrap;overflow-x:hidden;overflow-wrap:anywhere;}
+  .msg-body pre.md-source-block code,.preview-md pre.md-source-block code{display:block;white-space:inherit;overflow-wrap:anywhere;word-break:break-word;}
+  .msg-body pre.md-source-block code .token,.preview-md pre.md-source-block code .token{white-space:inherit;overflow-wrap:anywhere;word-break:inherit;}
   .provider-error-details{margin:12px 0 0;border:1px solid var(--border);border-radius:10px;background:var(--surface);overflow:hidden;}
   .provider-error-details>summary{cursor:pointer;color:var(--muted);font-size:12px;font-weight:600;padding:8px 12px;}
   .provider-error-details>pre{margin:0;border:0;border-top:1px solid var(--border);border-radius:0;max-height:220px;}
diff --git a/static/ui.js b/static/ui.js
index d530bb47..7db23b26 100644
--- a/static/ui.js
+++ b/static/ui.js
@@ -349,6 +349,7 @@ function _renderUserFencedBlocks(text){
     if(code.endsWith('\n')) code=code.slice(0,-1);
     const h=lang?`<div class="pre-header">${esc(lang)}</div>`:'';
     const langAttr=lang?` class="language-${esc(lang)}"`:'';
+    const preClass=/^(md|markdown|mdx)$/.test(lang)?' class="md-source-block"':'';
     if(lang==='diff'||lang==='patch'){
       const colored=esc(code).split('\n').map(line=>{
         if(line.startsWith('@@')) return `<span class="diff-line diff-hunk">${line}</span>`;
@@ -358,7 +359,7 @@ function _renderUserFencedBlocks(text){
       }).join('\n');
       stash.push(`${h}<pre class="diff-block"><code${langAttr}>${colored}</code></pre>`);
     } else {
-      stash.push(`${h}<pre><code${langAttr}>${esc(code)}</code></pre>`);
+      stash.push(`${h}<pre${preClass}><code${langAttr}>${esc(code)}</code></pre>`);
     }
     return lead+'\x00UF'+(stash.length-1)+'\x00';
   });
@@ -4856,6 +4857,7 @@ function renderMd(raw){
     } else {
       const h=lang?`<div class="pre-header">${esc(lang)}</div>`:'';
       const langAttr=lang?` class="language-${esc(lang)}"`:'';
+      const preClass=/^(md|markdown|mdx)$/.test(lang)?' class="md-source-block"':'';
       // For diff/patch blocks, wrap each line in a colored span
       if(lang==='diff'||lang==='patch'){
         const colored=esc(code.replace(/\n$/,'')).split('\n').map(line=>{
@@ -4881,10 +4883,10 @@ function renderMd(raw){
           const body=rows.slice(1).map(r=>'<tr>'+r.split(',').map(c=>`<td>${esc(c.trim())}</td>`).join('')+'</tr>').join('');
           _preBlock_stash.push(`${h}<div class="csv-table-wrap"><table class="csv-table"><thead><tr>${headers.map(h=>`<th>${esc(h)}</th>`).join('')}</tr></thead><tbody>${body}</tbody></table></div>`);
         } else {
-          _preBlock_stash.push(`${h}<pre><code${langAttr}>${esc(code.replace(/\n$/,''))}</code></pre>`);
+          _preBlock_stash.push(`${h}<pre${preClass}><code${langAttr}>${esc(code.replace(/\n$/,''))}</code></pre>`);
         }
       } else {
-        _preBlock_stash.push(`${h}<pre><code${langAttr}>${esc(code.replace(/\n$/,''))}</code></pre>`);
+        _preBlock_stash.push(`${h}<pre${preClass}><code${langAttr}>${esc(code.replace(/\n$/,''))}</code></pre>`);
       }
     }
     return lead+'\x00P'+(_preBlock_stash.length-1)+'\x00';
diff --git a/tests/test_1325_user_fenced_code.py b/tests/test_1325_user_fenced_code.py
index be918a4b..240e4560 100644
--- a/tests/test_1325_user_fenced_code.py
+++ b/tests/test_1325_user_fenced_code.py
@@ -126,7 +126,7 @@ class TestUserFencedBlocks:
     def test_four_backtick_outer_fence_preserves_inner_triple_fence(self):
         """User-message code fences should follow CommonMark fence-length matching too."""
         out = _run_user_render("````md\n```inner\nfoo\n```\n````")
-        assert out.count("<pre>") == 1
+        assert out.count("<pre class=\"md-source-block\">") == 1
         assert out.count("</pre>") == 1
         assert '<div class="pre-header">md</div>' in out
         assert "```inner" in out
diff --git a/tests/test_csv_table_rendering.py b/tests/test_csv_table_rendering.py
index a2afc3da..b86c413f 100644
--- a/tests/test_csv_table_rendering.py
+++ b/tests/test_csv_table_rendering.py
@@ -41,7 +41,9 @@ def test_csv_fence_fallback_for_insufficient_rows():
         src = f.read()
     fence_section = src[src.find("lang==='csv'"):src.find("lang==='csv'") + 800]
     assert 'rows.length>=2' in fence_section, "Should check for at least 2 rows"
-    assert '<pre><code' in fence_section, "Fallback should render as <pre><code>"
+    assert '<pre${preClass}><code${langAttr}>' in fence_section, (
+        "Fallback should render via the shared preClass-aware code-block template"
+    )
 
 
 def test_csv_media_file_handler():
diff --git a/tests/test_issue_code_syntax_highlight.py b/tests/test_issue_code_syntax_highlight.py
index d83617a6..d6dedd3d 100644
--- a/tests/test_issue_code_syntax_highlight.py
+++ b/tests/test_issue_code_syntax_highlight.py
@@ -17,9 +17,10 @@ def test_fenced_code_blocks_add_prism_language_class():
 def test_fenced_code_blocks_keep_existing_pre_header_layout():
     js = _read_ui_js()
     # The fenced code rendering was moved into the stash callback (#1154 fix).
-    # The template string now uses `lang` instead of `normalizedLang`.
-    assert '${h}<pre><code${langAttr}>${esc(code.replace(/\\n$/,' in js, (
-        "The syntax-highlight fix should preserve the existing fenced code block layout"
+    # The template string now uses `lang` instead of `normalizedLang`, and
+    # markdown-source fences may add a dedicated md-source-block class.
+    assert '${h}<pre${preClass}><code${langAttr}>${esc(code.replace(/\\n$/,' in js, (
+        "The syntax-highlight fix should preserve the shared fenced code block layout"
     )
     assert '<div class="code-block">' not in js, (
         "This fix should not introduce a new wrapper around fenced code blocks"
diff --git a/tests/test_markdown_source_code_blocks_wrap.py b/tests/test_markdown_source_code_blocks_wrap.py
new file mode 100644
index 00000000..14c5e603
--- /dev/null
+++ b/tests/test_markdown_source_code_blocks_wrap.py
@@ -0,0 +1,18 @@
+from pathlib import Path
+
+
+ROOT = Path(__file__).resolve().parents[1]
+UI_JS = (ROOT / "static" / "ui.js").read_text(encoding="utf-8")
+CSS = (ROOT / "static" / "style.css").read_text(encoding="utf-8")
+
+
+def test_markdown_source_code_blocks_get_dedicated_pre_class_in_both_renderers():
+    expected = "const preClass=/^(md|markdown|mdx)$/.test(lang)?' class=\"md-source-block\"':'';"
+
+    assert UI_JS.count(expected) >= 2, "Expected both fenced-code renderers to tag markdown source blocks"
+
+
+def test_markdown_source_code_blocks_have_desktop_wrap_css():
+    assert ".msg-body pre.md-source-block,.preview-md pre.md-source-block{white-space:pre-wrap;overflow-x:hidden;overflow-wrap:anywhere;}" in CSS
+    assert ".msg-body pre.md-source-block code,.preview-md pre.md-source-block code{display:block;white-space:inherit;overflow-wrap:anywhere;word-break:break-word;}" in CSS
+    assert ".msg-body pre.md-source-block code .token,.preview-md pre.md-source-block code .token{white-space:inherit;overflow-wrap:anywhere;word-break:inherit;}" in CSS
diff --git a/tests/test_renderer_js_behaviour.py b/tests/test_renderer_js_behaviour.py
index 2a84905a..0c63cc6e 100644
--- a/tests/test_renderer_js_behaviour.py
+++ b/tests/test_renderer_js_behaviour.py
@@ -397,7 +397,7 @@ class TestFencedCodeFenceLength:
             "That is much more correct than pretending"
         )
         out = _render(driver_path, src)
-        assert out.count("<pre>") == 1
+        assert out.count("<pre class=\"md-source-block\">") == 1
         assert out.count("</pre>") == 1
         assert '<div class="pre-header">md</div>' in out
         assert "```novelcrafter" in out
@@ -408,7 +408,7 @@ class TestFencedCodeFenceLength:
 
     def test_four_backtick_outer_fence_preserves_inner_triple_fence(self, driver_path):
         out = _render(driver_path, "````md\n```inner\nfoo\n```\n````\n")
-        assert out.count("<pre>") == 1
+        assert out.count("<pre class=\"md-source-block\">") == 1
         assert out.count("</pre>") == 1
         assert '<div class="pre-header">md</div>' in out
         assert "```inner" in out