Layout flash/jump when opening markdown files

[sporteka2]

Hi @Heroor, 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 <pre> 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 early CSS injection to hide raw content until rendering is complete. Two options:

Option A: Add run_at: document_start + early CSS

In manifest.json, add a separate content script that runs at document_start and injects CSS to hide raw <pre> content:

manifest.json — add to content_scripts array:

{
  "css": ["css/hide-raw.css"],
  "matches": ["*://*/*.md", "file://*/*.md"],
  "run_at": "document_start"
}

css/hide-raw.css:

/* Hide raw markdown until extension renders it */
pre {
  visibility: hidden !important;
}

/* When the extension replaces content, article is visible */
article {
  visibility: visible !important;
}

/* Fallback: if extension does not run, show content after 2s */
@keyframes reveal {
  to { visibility: visible; }
}
pre {
  animation: reveal 2s forwards;
}

Option B: Simpler — just set run_at: document_start

Change the existing content_scripts entry to run at document_start so the rendering happens before the browser paints the raw text:

"content_scripts": [
  {
    "js": ["js/content.js"],
    "css": ["css/content.css"],
    "matches": [...],
    "run_at": "document_start"
  }
]

With document_start, the script runs before any DOM is painted, so the raw markdown never appears visually.

Environment

• MD Reader version: 2.12.12
• Firefox 152.0 / Floorp 152.0 (Gecko-based browsers)
• OS: Linux Mint 22.3 (1920x1080)

Would you consider implementing one of these options? Happy to test a pre-release build.

Thanks!

[Heroor]

Markdown Reader for Firefox has been updated to version 2.17.0. If you encounter any issues, please feel free to let me know!