     node.closest("scrollbar,scrollcorner")
        )) ||
      content.XULElement.isInstance(node.closest("treechildren"))
    ) {
      return true;
    }
    return false;
  }

  isScrollableElement(aNode) {
    let content = aNode.ownerGlobal;
    if (content.HTMLElement.isInstance(aNode)) {
      return !content.HTMLSelectElement.isInstance(aNode) || aNode.multiple;
    }

    return content.XULElement.isInstance(aNode);
  }

  computeWindowScrollDirection(global) {
    if (!global.scrollbars.visible) {
      return null;
    }
    if (global.scrollMaxX != global.scrollMinX) {
      return global.scrollMaxY != global.scrollMinY ? "NSEW" : "EW";
    }
    if (global.scrollMaxY != global.scrollMinY) {
      return "NS";
    }
    return null;
  }

  computeNodeScrollDirection(node) {
    if (!this.isScrollableElement(node)) {
      return null;
    }

    let global = node.ownerGlobal;

    // this is a list of overflow property values that allow scrolling
    const scrollingAllowed = ["scroll", "auto"];

    let cs = global.getComputedStyle(node);
    let overflowx = cs.getPropertyValue("overflow-x");
    let overflowy = cs.getPropertyValue("overflow-y");
    // we already discarded non-multiline selects so allow vertical
    // scroll for multiline ones directly without checking for a
    // overflow property
    let scrollVert =
      node.scrollTopMax &&
      (global.HTMLSelectElement.isInstance(node) ||
        scrollingAllowed.includes(overflowy));

    // do not allow horizontal scrolling for select elements, it leads
    // to visual artifacts and is not the expected behavior anyway
    if (
      !global.HTMLSelectElement.isInstance(node) &&
      node.scrollLeftMin != node.scrollLeftMax &&
      scrollingAllowed.includes(overflowx)
    ) {
      return scrollVert ? "NSEW" : "EW";
    }

    if (scrollVert) {
      return "NS";
    }

    return null;
  }

  findNearestScrollableElement(aNode) {
    // go upward in the DOM and find any parent element that has a overflow
    // area and can therefore be scrolled
    this._scrollable = null;
    for (let node = aNode; node; node = node.flattenedTreeParentNode) {
      // do not use overflow based autoscroll for <html> and <body>
      // Elements or non-html/non-xul elements such as svg or Document nodes
      // also make sure to skip select elements that are not multiline
      let direction = this.computeNodeScrollDirection(node);
      if (direction) {
        this._scrolldir = direction;
        this._scrollable = node;
        break;
      }
    }

    if (!this._scrollable) {
      let direction = this.computeWindowScrollDirection(aNode.ownerGlobal);
      if (direction) {
        this._scrolldir = direction;
        this._scrollable = aNode.ownerGlobal;
      } else if (aNode.ownerGlobal.frameElement) {
        // Note, in case of out of process iframes frameElement is null, and
        // a caller is supposed to communicate to iframe's parent on its own to
        // support cross process scrolling.
        this.findNearestScrollableElement(aNode.ownerGlobal.frameElement);
      }
    }
  }

  async startScroll(event) {
    this.findNearestScrollableElement(event.originalTarget);
    if (!this._scrollable) {
      this.sendAsyncMessage("Autoscroll:MaybeStartInParent", {
        browsingContextId: this.browsingContext.id,
        screenX: event.screenX,
        screenY: event.screenY,
      });
      return;
    }

    let content = event.originalTarget.ownerGlobal;

    // In some configurations like Print Preview, content.performance
    // (which we use below) is null. Autoscrolling is broken in Print
    // Preview anyways (see bug 1393494), so just don't start it at all.
    if (!content.performance) {
      return;
    }

    let domUtils = content.windowUtils;
    let scrollable = this._scrollable;
    if (scrollable instanceof Ci.nsIDOMWindow) {
      // getViewId() needs an element to operate on.
      scrollable = scrollable.document.documentElement;
    }
    this._scrollId = null;
    try {
      this._scrollId = domUtils.getViewId(scrollable);
    } catch (e) {
      // No view ID - leave this._scrollId as null. Receiving side will check.
    }
    let presShellId = domUtils.getPresShellId();
    let { autoscrollEnabled, usingApz } = await this.sendQuery(
      "Autoscroll:Start",
      {
        scrolldir: this._scrolldir,
        screenXDevPx: event.screenX * content.devicePixelRatio,
        screenYDevPx: event.screenY * content.devicePixelRatio,
        scrollId: this._scrollId,
        presShellId,
        browsingContext: this.browsingContext,
      }
    );
    if (!autoscrollEnabled) {
      this._scrollable = null;
      return;
    }

    this.document.addEventListener("mousemove", this, {
      capture: true,
      mozSystemGroup: true,
    });
    this.document.addEventListener("mouseup", this, {
      capture: true,
      mozSystemGroup: true,
    });
    this.document.addEventListener("pagehide", this, true);

    this._startX = event.screenX;
    this._startY = event.screenY;
    this._screenX = event.screenX;
    this._screenY = event.screenY;
    this._scrollErrorX = 0;
    this._scrollErrorY = 0;
    this._autoscrollHandledByApz = usingApz;

    if (!usingApz) {
      // If the browser didn't hand the autoscroll off to APZ,
      // scroll here in the main thread.
      this.startMainThreadScroll();
    } else {
      // Even if the browser did hand the autoscroll to APZ,
      // APZ might reject it in which case it will notify us
      // and we need to take over.
      Services.obs.addObserver(this.observer, "autoscroll-rejected-by-apz");
    }

    if (Cu.isInAutomation) {
      Services.obs.notifyObservers(content, "autoscroll-start");
    }
  }

  startMainThreadScroll() {
    let content = this.document.defaultView;
    this._lastFrame = content.performance.now();
    content.requestAnimationFrame(this.autoscrollLoop);
  }

  stopScroll() {
    if (this._scrollable) {
      this._scrollable.mozScrollSnap();
      this._scrollable = null;

      this.document.removeEventListener("mousemove", this, {
        capture: true,
        mozSystemGroup: true,
      });
      this.document.removeEventListener("mouseup", this, {
        capture: true,
        mozSystemGroup: true,
      });
      this.document.removeEventListener("pagehide", this, true);
      if (this._autoscrollHandledByApz) {
        Services.obs.removeObserver(
          this.observer,
          "autoscroll-rejected-by-apz"
        );
      }
    }
  }

  accelerate(curr, start) {
    const speed = 12;
    var val = (curr - start) / speed;

    if (val > 1) {
      return val * Math.sqrt(val) - 1;
    }
    if (val < -1) {
      return val * Math.sqrt(-val) + 1;
    }
    return 0;
  }

  roundToZero(num) {
    if (num > 0) {
      return Math.floor(num);
    }
    return Math.ceil(num);
  }

  autoscrollLoop(timestamp) {
    if (!this._scrollable) {
      // Scrolling has been canceled
      return;
    }

    // avoid long jumps when the browser hangs for more than
    // |maxTimeDelta| ms
    const maxTimeDelta = 100;
    var timeDelta = Math.min(maxTimeDelta, timestamp - this._lastFrame);
    // we used to scroll |accelerate()| pixels every 20ms (50fps)
    var timeCompensation = timeDelta / 20;
    this._lastFrame = timestamp;

    var actualScrollX = 0;
    var actualScrollY = 0;
    // don't bother scrolling vertically when the scrolldir is only horizontal
    // and the other way around
    if (this._scrolldir != "EW") {
      var y = this.accelerate(this._screenY, this._startY) * timeCompensation;
      var desiredScrollY = this._scrollErrorY + y;
      actualScrollY = this.roundToZero(desiredScrollY);
      this._scrollErrorY = desiredScrollY - actualScrollY;
    }
    if (this._scrolldir != "NS") {
      var x = this.accelerate(this._screenX, this._startX) * timeCompensation;
      var desiredScrollX = this._scrollErrorX + x;
      actualScrollX = this.roundToZero(desiredScrollX);
      this._scrollErrorX = desiredScrollX - actualScrollX;
    }

    this._scrollable.scrollBy({
      left: actualScrollX,
      top: actualScrollY,
      behavior: "instant",
    });

    this._scrollable.ownerGlobal.requestAnimationFrame(this.autoscrollLoop);
  }

  canStartAutoScrollWith(event) {
    if (
      !event.isTrusted ||
      event.defaultPrevented ||
      event.button !== 1 ||
      event.clickEventPrevented()
    ) {
      return false;
    }

    for (const modifier of ["shift", "alt", "ctrl", "meta"]) {
      if (
        event[modifier + "Key"] &&
        Services.prefs.getBoolPref(
          `general.autoscroll.prevent_to_start.${modifier}Key`,
          false
        )
      ) {
        return false;
      }
    }
    return true;
  }

  handleEvent(event) {
    switch (event.type) {
      case "mousemove":
        this._screenX = event.screenX;
        this._screenY = event.screenY;
        break;
      case "mousedown":
        if (
          this.canStartAutoScrollWith(event) &&
          !this._scrollable &&
          !this.isAutoscrollBlocker(event)
        ) {
          this.startScroll(event);
        }
      // fallthrough
      case "mouseup":
        if (
          this._scrollable &&
          Services.prefs.getBoolPref("general.autoscroll", false)
        ) {
          // Middle mouse click event shouldn't be fired in web content for
          // compatibility with Chrome.
          event.preventClickEvent();
        }
        break;
      case "pagehide":
        if (this._scrollable) {
          var doc = this._scrollable.ownerDocument || this._scrollable.document;
          if (doc == event.target) {
            this.sendAsyncMessage("Autoscroll:Cancel");
            this.stopScroll();
          }
        }
        break;
    }
  }

  receiveMessage(msg) {
    let data = msg.data;
    switch (msg.name) {
      case "Autoscroll:MaybeStart":
        for (let child of this.browsingContext.children) {
          if (data.browsingContextId == child.id) {
            this.startScroll({
              screenX: data.screenX,
              screenY: data.screenY,
              originalTarget: child.embedderElement,
            });
            break;
          }
        }
        break;
      case "Autoscroll:Stop": {
        this.stopScroll();
        break;
      }
    }
  }

  rejectedByApz(data) {
    // The caller passes in the scroll id via 'data'.
    if (data == this._scrollId) {
      this._autoscrollHandledByApz = false;
      this.startMainThreadScroll();
      Services.obs.removeObserver(this.observer, "autoscroll-rejected-by-apz");
    }
  }
}

class AutoScrollObserver {
  constructor(actor) {
    this.actor = actor;
  }

  observe(subject, topic, data) {
    if (topic === "autoscroll-rejected-by-apz") {
      this.actor.rejectedByApz(data);
    }
  }
}
PK