This component makes it possible for a <video> element on a web page to be played within an always-on-top video player.

This documentation covers the architecture and inner workings of both the mechanism that displays the <video> in the always-on-top video player, as well as the mechanism that displays the Picture-in-Picture toggle that overlays <video> elements, which is the primary method for launching the feature.

High-level overview

The following diagram tries to illustrate the subcomponents, and how they interact with one another.


Let’s suppose that the user has loaded a document with a <video> in it, and they decide to open it in a Picture-in-Picture window. What happens?

First the PictureInPictureToggleChild component notices when <video> elements are added to the DOM, and monitors the mouse as it moves around the document. Once the mouse intersects a <video>, PictureInPictureToggleChild causes the Picture-in-Picture toggle to appear on that element.

If the user clicks on that toggle, then the PictureInPictureToggleChild dispatches a chrome-only MozTogglePictureInPicture event on the video, which is handled by the PictureInPictureLauncherChild actor for that document. The reason for the indirection via the event is that the media context menu can also trigger Picture-in-Picture by dispatching the same event on the video. Upon handling the event, the PictureInPictureLauncherChild actor then sends a PictureInPicture:Request message to the parent process. The parent process opens up the always-on-top player window, with a remote <xul:browser> that runs in the same content process as the original <video>. The parent then sends a message to the player window’s remote <xul:browser> loaded in the player window. A PictureInPictureChild actor is instantiated for the empty document loaded inside of the player window browser. This PictureInPictureChild actor constructs its own <video> element, and then tells Gecko to clone the frames from the original <video> to the newly created <video>.

At this point, the video is displaying in the Picture-in-Picture player window.

Next, we’ll discuss the individual subcomponents, and how they operate at a more detailed level.

The Picture-in-Picture toggle

One of the primary challenges faced when developing this feature was the fact that, in practice, mouse events tend not to reach <video> elements. This is usually because the <video> element is contained within a hierarchy of other DOM elements that are capturing and handling any events that come down. This often occurs on sites that construct their own video controls. This is why we cannot simply use a mouseover event handler on the <video> UAWidget - on sites that do the event capturing, we’ll never receive those events and the toggle will not be accessible.

Other times, the problem is that the video is overlaid with a semi or fully transparent element which captures any mouse events that would normally be dispatched to the underlying <video>. This can occur, for example, on sites that want to display an overlay when the video is paused.

To work around this problem, the PictureInPictureToggleChild actor class samples the latest mousemove event every MOUSEMOVE_PROCESSING_DELAY_MS milliseconds, and then calls nsIDOMWindowUtils.nodesFromRect with the aOnlyVisible argument to get the full list of visible nodes that exist underneath a 1x1 rect positioned at the mouse cursor.

If a <video> is in that list, then we reach into its shadow root, and update some attributes to tell it to maybe show the toggle.

The underlying UAWidget for the video is defined in videocontrols.js, and ultimately chooses whether or not to display the toggle based on the following heuristics:

  1. Is the video less than 45 seconds?

  2. Is either the width or the height of the video less than 160px?

  3. Is the video silent?

If any of the above is true, the underlying UAWidget will hide the toggle, since it’s unlikely that the user will want to pop the video out into an always-on-top player window.

Video registration

Sampling the latest mousemove event every MOUSEMOVE_PROCESSING_DELAY_MS is not free, computationally speaking, so we only do this if there are one or more <video> elements visible on the page. We use an IntersectionObserver to notice when there is a <video> within the viewport, and if there are 1 or more <video> elements visible, then we start sampling the mousemove event.

Videos are added to the IntersectionObserver when they are added to the DOM by listening for the UAWidgetSetupOrChange event. This is considered being “registered”.


PictureInPictureChild.sys.mjs contains a WeakMap mapping document’s to various information that PictureInPictureToggleChild wants to retain for the lifetime of that document. For example, whether or not we’re in the midst of handling the user clicking down on their pointer device. Any state that needs to be remembered should be added to the docState WeakMap.

Clicking on the toggle

If the user clicks on the Picture-in-Picture toggle, we don’t want the underlying webpage to know that this happened, since this could result in unexpected behaviour, like a page navigation (for example, if the <video> is a long-running advertisement that navigates upon click).

To accomplish this, we listen for all events fired on a mouse click on the root window during the capturing phase. This allows us to handle the events before they are dispatched to content.

The first event that is fired, pointerdown, is captured, and we check the docState to see whether or not we’re showing a toggle on any videos. If so, we check the coordinates of that toggle against the coordinates of the pointerdown event to determine if the user is clicking on the toggle. If so, we set a flag in the docState so that any subsequent events from the click (like mousedown, mouseup, pointerup, click) are captured and suppressed. If the pointerdown event didn’t occur within a toggle, we let the events pass through as normal.

If we determine that the click has occurred on the toggle, a MozTogglePictureInPicture event is dispatched on the underlying <video>. This event is handled by the separate PictureInPictureLauncherChild class.


A small actor class whose only responsibility is to tell the parent process to open an always-on-top-window by sending a PictureInPicture:Request message to its parent actor.

Currently, this only occurs when a chrome-only MozTogglePictureInPicture event is dispatched by the PictureInPictureToggleChild when the user clicks the Picture-in-Picture toggle button or uses the context-menu.


The PictureInPictureChild actor class will run in a content process containing a video, and is instantiated when the player window’s player.js script runs its initialization. A PictureInPictureChild maps an individual <video> to a player window instance. It creates an always-on-top window, and sets up a new <video> inside of this window to clone frames from another <video> (which will be in the same process, and have its own PictureInPictureChild). Creating this window also causes the new PictureInPictureChild to be created. This instance will monitor the originating <video> for changes, and to receive commands from the player window if the user wants to control the <video>.


This module runs in the parent process, and is also the scope where all PictureInPictureParent instances reside. PictureInPicture.sys.mjs’s job is to send and receive messages from PictureInPictureChild instances, and to react appropriately.

Critically, PictureInPicture.sys.mjs is responsible for opening up the always-on-top player window, and passing the relevant information about the <video> to be displayed to it.

The Picture-in-Picture player window

The Picture-in-Picture player window is a chrome-privileged window that loads an XHTML document. That document contains a remote <browser> element which is repurposed during window initialization to load in the same content process as the originating <video>.

The player window is where the player controls are defined, like “Play” and “Pause”. When the user interacts with the player controls, a message is sent down to the appropriate PictureInPictureChild to call the appropriate method on the underlying <video> element in the originating tab.

Cloning the video frames

While it appears as if the video is moving from the original <video> element to the player window, what’s actually occurring is that the video frames are being cloned to the player window <video> element. This cloning is done at the platform level using a privileged method on the <video> element: cloneElementVisually.


Promise<void> video.cloneElementVisually(otherVideo);

This will clone the frames being decoded for video and display them on the otherVideo element as well. The returned Promise resolves once the cloning has successfully started.


void video.stopCloningElementVisually();

If video is being cloned visually to another element, calling this method will stop the cloning.


boolean video.isCloningElementVisually;

A read-only value that returns true if video is being cloned visually.

Site-specific video wrappers

A site-specific video wrapper allows for the creation of custom scripts that the Picture-in-Picture component can utilize when videos are loaded in specific domains. Currently, some uses of video wrappers include:

  • Integration of captions and subtitles support on certain video streaming sites

  • Fixing inconsistent video behaviour when using Picture-in-Picture controls

  • Hiding the Picture-in-Picture toggle for videos on particular areas of a page, given a URL (rather than hiding the toggle for all videos on a page)

PictureInPictureChildVideoWrapper and videoWrapperScriptPath

PictureInPictureChildVideoWrapper is a special class that represents a video wrapper. It is defined in PictureInPictureChild.sys.mjs and maps to a videoWrapperScriptPath, which is the path of the custom wrapper script to use. videoWrapperScriptPath is defined in browser/extensions/pictureinpicture/data/picture_in_picture_overrides.js for a domain, and custom wrapper scripts are defined in browser/extensions/pictureinpicture/video-wrappers.

If a videoWrapperScriptPath is detected while initializing the Picture-in-Picture toggle or window, we immediately create a new instance of PictureInPictureChildVideoWrapper based on the given path, allowing us to run our custom scripts.


See the full list of methods at API References.


Performing video control operations on the originating video requires executing code in the browser content. For security reasons, we utilize a sandbox to isolate these operations and prevent direct access to PictureInPictureChild. In other words, we run content code within the sandbox itself. However, it is necessary to waive xray vision so that we can execute the video control operations. This is done by reading the wrapper’s .wrappedJSObject property.

Adding a new site-specific video wrapper

Creating a new wrapper script file

Add a new JS file for the new video wrapper in browser/extensions/pictureinpicture/video-wrappers. The file must meet several requirements to get the wrapper working.

Script file requirements:

  • Defined class PictureInPictureVideoWrapper

  • Assigned this.PictureInPictureVideoWrapper = PictureInPictureVideoWrapper

PictureInPictureVideoWrapper class requirements:

Overriden method requirements:

  • Return value with a type that corresponds to validateRetVal in PictureInPictureChildVideoWrapper.#callWrapperMethod()

Below is an example of a script file mock-wrapper.js that overrides an existing method setMuted() in PictureInPictureChildVideoWrapper:

// sample file `mock-wrapper.js`
class PictureInPictureVideoWrapper {
   setMuted(video, shouldMute) {
      if (video.muted !== shouldMute) {
         let muteButton = document.querySelector("#player .mute-button");
         if (muteButton) {
         } else {
            video.muted = shouldMute;

this.PictureInPictureVideoWrapper = PictureInPictureVideoWrapper


If a new PictureInPictureChildVideoWrapper video control method is needed, see Adding a new video control method.

Declaring videoWrapperScriptPath

Declare a property videoWrapperScriptPath for the site at browser/extensions/pictureinpicture/data/picture_in_picture_overrides.js:

someWebsite: {
  "https://*.somewebsite.com/*": {
    videoWrapperScriptPath: "video-wrappers/mock-wrapper.js",

In this example, the URL pattern https://*.somewebsite.com/* is provided for a site named someWebsite. Picture-in-Picture checks for any overrides upon initialization, and it will load scripts specified by videoWrapperScriptPath. The scripts located at video-wrappers/mock-wrapper.js will therefore run whenever we view a video from a URL matching somewebsite.com.

Registering the new wrapper in moz.build

We should update browser/extensions/pictureinpicture/moz.build by adding the path of the newly created wrapper:

FINAL_TARGET_FILES.features["pictureinpicture@mozilla.org"]["video-wrappers"] += [

As expected for any moz.build file, order matters. Registered paths should be listed in alphabetical order. Otherwise, the build will fail.

Adding a new video control method

If none of the existing overridable methods in PictureInPictureChildVideoWrapper are applicable for a bug fix or feature enhancement, we can create a new one by calling #callWrapperMethod(). Below is an example of how we would define a new overridable method setMuted():

// class PictureInPictureChildVideoWrapper in PictureInPictureChild.sys.mjs
setMuted(video, shouldMute) {
    return this.#callWrapperMethod({
        name: "setMuted",
        args: [video, shouldMute],
        fallback: () => {
            video.muted = shouldMute;
        validateRetVal: retVal => retVal == null,

The new method passes to #callWrapperMethod():

  1. The method name

  2. The expected arguments that a wrapper script may use

  3. A fallback function

  4. A conditional expression that validates the return value

The fallback function only executes if a wrapper script fails or if the method is not overriden. validateRetVal checks the type of the return value and ensures it matches the expected type. If there is no return value, simply validate if type is null.


Generic method names are preferred so that they can be used for any video wrapper. For example: instead of naming a method updateCaptionsContainerForSiteA(), use updateCaptionsContainer().

Using the new video control method

Once the new method is defined, it can be used throughout PictureInPictureChild.sys.mjs. In the current example, we call PictureInPictureChildVideoWrapper.setMuted() to mute or unmute a video. this.videoWrapper is an instance of PictureInPictureChildVideoWrapper:

// class PictureInPictureChild in PictureInPictureChild.sys.mjs
mute() {
    let video = this.getWeakVideo();
    if (video && this.videoWrapper) {
        this.videoWrapper.setMuted(video, true);

unmute() {
    let video = this.getWeakVideo();
    if (video && this.videoWrapper) {
        this.videoWrapper.setMuted(video, false);

Testing site-specific video wrappers

Automated Tests

Automated tests for site specific wrappers are currently limited. New tests can be made in browser/extensions/pictureinpicture/tests/browser to ensure general functionality, but these are restricted to Firefox Nightly and do not test functionality on specific sites.

Some challenges with writing tests include:

  • Accessing DRM content

  • Log-in credentials if a site requires a user account

  • Detecting modifications to a web page or video player that render a wrapper script obsolete

Manual Tests

The go-to approach right now is to test video wrappers manually, in tandem with reviews provided by the phabricator group #pip-reviewers. Below are some questions that reviewers will consider:

  • Does Picture-in-Picture crash or freeze?

  • Does the wrapper work on Windows, MacOS, and Linux?

  • Do Picture-in-Picture features work as expected? (Picture-in-Picture toggle, text tracks, video controls, etc.)

  • Do existing automated tests work as they should?


DRM content may not load for all local Firefox builds. One possible solution is to test the video wrapper in a try build (ex. Linux). Depending on the changes made, we may also require the script to run under a temporary pref such as media.videocontrols.picture-in-picture.WIP.someWebsiteWrapper for the purpose of testing changes in Firefox Nightly.

API References