Fix scroll over mermaid frame (#21925)

When starting a scroll while the mouse is over a mermaid diagram, the
scroll sometimes propagates to the iframe, preventing the parent page
from scrolling. Fix this by disabling scroll inside the iframe. This is
not a problem because those frames are never meant to scroll. Bug seems
to affect Firefox only.


![scroll](https://user-images.githubusercontent.com/115237/203847578-6831e3c8-9df4-4577-8501-822fb9ea1278.gif)

web_src/js/markup/mermaid.js

@@ -3,7 +3,7 @@ const {mermaidMaxSourceCharacters} = window.config;
 
 const iframeCss = `
   :root {color-scheme: normal}
-  body {margin: 0; padding: 0}
+  body {margin: 0; padding: 0; overflow: hidden}
   #mermaid {display: block; margin: 0 auto}
 `;