diff --git a/src/web/public/styles.css b/src/web/public/styles.css
index 18e9e56b..89fdc70b 100644
--- a/src/web/public/styles.css
+++ b/src/web/public/styles.css
@@ -12549,12 +12549,16 @@ html:is([data-skin="paper-gray"], [data-skin="solarized-light"], [data-skin="cat
    No width cap. The window is whatever the user drags or expands it to, so the
    document must reflow to fill it; the fixed 960px reading column this rule
    shipped before the resize grip existed stops reflow once the window is wider
-   than the cap. Pinned by test/file-preview-window.test.ts. */
+   than the cap. --rv-content-max: none lets ASCII-diagram blocks
+   (.rv-text .rv-diagram-wrap, capped by that variable with a 720px fallback
+   meant for the Response Viewer) follow the rest of the document too. Pinned by
+   test/file-preview-window.test.ts. */
 
 .file-preview-body > .file-preview-md {
   padding: 1rem 1.25rem 2rem;
   font-size: 15px;
   line-height: 1.55;
+  --rv-content-max: none;
 }
 
 /* Relative links in the document are rebased onto a.rv-path so the Response
diff --git a/test/file-preview-window.test.ts b/test/file-preview-window.test.ts
index 08842771..d69a1cf0 100644
--- a/test/file-preview-window.test.ts
+++ b/test/file-preview-window.test.ts
@@ -391,6 +391,9 @@ describe('file viewer rendered markdown width', () => {
     const rvBase = stylesCss.match(/\.rv-text,\s*\.response-viewer-body > :not\(\.rv-message\) \{[^}]*\}/);
     expect(rvBase, 'the shared prose rule').not.toBeNull();
     expect(rvBase![0], 'no width cap on the shared prose rule').not.toMatch(/max-width/);
+    // ASCII-diagram blocks cap themselves at var(--rv-content-max, 720px), which only the
+    // Response Viewer sets; the document lifts it so a diagram follows the window too.
+    expect(mdRule![0], 'diagram blocks follow the window').toMatch(/--rv-content-max:\s*none;/);
   });
 });