Layout flash/jump when opening markdown files — run_at default causes visible raw text

[sporteka2]

Hi @Cimbali, thank you for this great extension!

I have noticed a layout "jump" (flash of unstyled content) when opening markdown files. The raw markdown text is briefly visible before the extension replaces it with the rendered HTML.

Root cause

The content_scripts in manifest.json does not specify run_at, so it defaults to document_idle. This means:

1. Firefox loads and displays the raw markdown text
2. The content script runs after the page is idle
3. The rendered article replaces the raw text → visible layout shift

Proposed fix

Add "run_at": "document_start" to the content_scripts entry in manifest.json, so the script runs before the browser paints the raw text.

Alternatively, inject early CSS via a separate content script at document_start to hide <pre> content until the extension renders it:

/* Hide raw markdown until extension renders it */
pre { visibility: hidden !important; }
article { visibility: visible !important; }
@keyframes reveal { to { visibility: visible; } }
pre { animation: reveal 2s forwards; }

Environment

• Markdown Viewer Webext 1.0.1
• Firefox 152.0
• OS: Linux Mint 22.3 (1920x1080)

This is the same issue reported for MD Reader: md-reader/md-reader#156

Would you consider implementing one of these options?

Thanks!