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._spreadMode === SpreadMode.NONE) {
          break; 
        }
        const parity = this._spreadMode - 1;
        if (previous && currentPageNumber % 2 !== parity) {
          break; 
        } else if (!previous && currentPageNumber % 2 === parity) {
          break; 
        }
        const { views } = this._getVisiblePages(),
          expectedId = previous ? currentPageNumber - 1 : currentPageNumber + 1;
        for (const { id, percent, widthPercent } of views) {
          if (id !== expectedId) {
            continue;
          }
          if (percent > 0 && widthPercent === 100) {
            return 2;
          }
          break;
        }
        break;
      }
    }
    return 1;
  }
  /**
   * Go to the next page, taking scroll/spread-modes into account.
   * @returns {boolean} Whether navigation occured.
   */
  nextPage() {