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)

}
          } else {
            for (let i = currentIndex + 1, ii = numPages; i < ii; i++) {
              const currentId = yArray[i],
                expectedId = yArray[i - 1] + 1;
              if (currentId > expectedId) {
                return expectedId - currentPageNumber;
              }
            }
          }
          
          if (previous) {
            const firstId = yArray[0];
            if (firstId < currentPageNumber) {
              return currentPageNumber - firstId + 1;
            }
          } else {
            const lastId = yArray[numPages - 1];
            if (lastId > currentPageNumber) {
              return lastId - currentPageNumber + 1;
            }
          }
          break;
        }
        break;
      }
      case ScrollMode.HORIZONTAL: {
        break;
      }
      case ScrollMode.VERTICAL: {