I was wondering if WebGL is considered stable enough to be enabled by default. It makes some PDFs render much faster and it has been off by default for a while now, so we have had quite some time to test it.

If we decide to enable WebGL by default, we should not forget to update https://github.com/mozilla/pdf.js/blob/master/extensions/chromium/preferences_schema.json#L39 as well.

/cc @yurydelendik 
/cc @Snuffleupagus (because IIRC you were testing WebGL too)

if (!this.pdfDocument || !this._buffer) {
      return false;
    }
    if (
      !(
        Number.isInteger(pageNumber) &&
        pageNumber > 0 &&
        pageNumber <= this.pagesCount
      )
    ) {
      console.error(
        `${this._name}.isPageCached: "${pageNumber}" is not a valid page.`
      );
      return false;
    }
    const pageView = this._pages[pageNumber - 1];
    if (!pageView) {
      return false;
    }
    return this._buffer.has(pageView);
  }
  cleanup() {