old_contents: function addDrageScroll() {
      let flag = false;
      let lastClientX = 0;
      let lastClientY = 0;
      document.getElementById('image-container').addEventListener("mousedown", e => {
          if (e.button == 2) flag = true;
          lastClientX = e.clientX;
          lastClientY = e.clientY;
      });
      document.body.addEventListener("mouseup", e => {
          flag = false;
      });
      document.body.addEventListener("mousemove", e => {
          if (flag) {
              let x = window.scrollX -
                  (-lastClientX + (lastClientX = e.clientX));
              let y = window.scrollY -
                  (-lastClientY + (lastClientY = e.clientY));
              window.scrollTo(x, y);
          }
      });
  }
new_contents: function addDrageScroll() {
      let flag = false;
      let lastClientX = 0;
      let lastClientY = 0;
      document.getElementById('image-container').addEventListener("mousedown", e => {
          if (e.button == 2) flag = true;
          lastClientX = e.clientX;
          lastClientY = e.clientY;
      });
      document.body.addEventListener("mouseup", e => {
          flag = false;
      });
      document.body.addEventListener("mousemove", e => {
          if (flag) {
              let x = window.scrollX -
                  (-lastClientX + (lastClientX = e.clientX));
              let y = window.scrollY -
                  (-lastClientY + (lastClientY = e.clientY));
              window.scrollTo(x, y);
          }
      });
      window.addEventListener('contextmenu', e => {
          e.stopImmediatePropagation()
      }, true);
  }
subject: fix #413