message: Disable `view-markdown-source` where it already exists
diff: @@ -6,34 +6,43 @@ import {CodeIcon, FileIcon} from '@primer/octicons-react';
  import features from '.';
  async function init(): Promise<void> {
  - const isPlain = new URLSearchParams(location.search).get('plain') === '1';
  + if (select.exists('[href^="?plain=1"]')) {
  + throw new Error('GitHub has implemented this feature. Please report this issue here https://github.com/sindresorhus/refined-github/pull/4837');
  + }
  +
  select('#raw-url')!.closest('.d-flex')!.prepend(
  - <div className="BtnGroup rgh-view-markdown-source mr-1">
  + <div className="BtnGroup rgh-view-markdown-source">
  <a
  href="?plain=1"
  - data-pjax="#repo-content-pjax-container"
  - className="btn btn-sm BtnGroup-item tooltipped tooltipped-nw"
  - aria-label="Display the source"
  + aria-label="Display the source blob"
  + role="button"
  + data-view-component="true"
  + className="source tooltipped tooltipped tooltipped-n btn-sm btn BtnGroup-item"
  >
  <CodeIcon/>
  </a>
  <a
  href={location.pathname}
  - data-pjax="#repo-content-pjax-container"
  - className="btn btn-sm BtnGroup-item tooltipped tooltipped-nw"
  - aria-label="Display the rendered file"
  + aria-label="Display the rendered blob"
  + role="button"
  + data-view-component="true"
  + className="rendered tooltipped tooltipped tooltipped-n btn-sm btn BtnGroup-item"
  >
  <FileIcon/>
  </a>
  </div>,
  );
  + const isPlain = new URLSearchParams(location.search).get('plain') === '1';
  select('.rgh-view-markdown-source')!.children[isPlain ? 0 : 1].classList.add('selected');
  }
  void features.add(__filebasename, {
  include: [
  - () => pageDetect.isSingleFile() && /\.(mdx?|mkdn?|mdwn|mdown|markdown|litcoffee|rst)$/.test(location.pathname),
  + pageDetect.isSingleFile,
  + ],
  + asLongAs: [
  + () => /\.(mdx|mdwn|litcoffee|rst)$/.test(location.pathname),
  ],
  deduplicate: '.rgh-view-markdown-source', // #3945
  init,
  