Function onScrollWheel

static/editor.md/lib/codemirror/lib/codemirror.js:3840–3907

(cm, e)

  };

  function onScrollWheel(cm, e) {
    var delta = wheelEventDelta(e), dx = delta.x, dy = delta.y;

    var display = cm.display, scroll = display.scroller;
    // Quit if there's nothing to scroll here
    if (!(dx && scroll.scrollWidth > scroll.clientWidth ||
          dy && scroll.scrollHeight > scroll.clientHeight)) return;

    // Webkit browsers on OS X abort momentum scrolls when the target
    // of the scroll event is removed from the scrollable element.
    // This hack (see related code in patchDisplay) makes sure the
    // element is kept around.
    if (dy && mac && webkit) {
      outer: for (var cur = e.target, view = display.view; cur != scroll; cur = cur.parentNode) {
        for (var i = 0; i < view.length; i++) {
          if (view[i].node == cur) {
            cm.display.currentWheelTarget = cur;
            break outer;
          }
        }
      }
    }

    // On some browsers, horizontal scrolling will cause redraws to
    // happen before the gutter has been realigned, causing it to
    // wriggle around in a most unseemly way. When we have an
    // estimated pixels/delta value, we just handle horizontal
    // scrolling entirely here. It'll be slightly off from native, but
    // better than glitching out.
    if (dx && !gecko && !presto && wheelPixelsPerUnit != null) {
      if (dy)
        setScrollTop(cm, Math.max(0, Math.min(scroll.scrollTop + dy * wheelPixelsPerUnit, scroll.scrollHeight - scroll.clientHeight)));
      setScrollLeft(cm, Math.max(0, Math.min(scroll.scrollLeft + dx * wheelPixelsPerUnit, scroll.scrollWidth - scroll.clientWidth)));
      e_preventDefault(e);
      display.wheelStartX = null; // Abort measurement, if in progress
      return;
    }

    // 'Project' the visible viewport to cover the area that is being
    // scrolled into view (if we know enough to estimate it).
    if (dy && wheelPixelsPerUnit != null) {
      var pixels = dy * wheelPixelsPerUnit;
      var top = cm.doc.scrollTop, bot = top + display.wrapper.clientHeight;
      if (pixels < 0) top = Math.max(0, top + pixels - 50);
      else bot = Math.min(cm.doc.height, bot + pixels + 50);
      updateDisplaySimple(cm, {top: top, bottom: bot});
    }

    if (wheelSamples < 20) {
      if (display.wheelStartX == null) {
        display.wheelStartX = scroll.scrollLeft; display.wheelStartY = scroll.scrollTop;
        display.wheelDX = dx; display.wheelDY = dy;
        setTimeout(function() {
          if (display.wheelStartX == null) return;
          var movedX = scroll.scrollLeft - display.wheelStartX;
          var movedY = scroll.scrollTop - display.wheelStartY;
          var sample = (movedY && display.wheelDY && movedY / display.wheelDY) ||
            (movedX && display.wheelDX && movedX / display.wheelDX);