Asynchronous Panning and Zooming

This document is a work in progress. Some information may be missing or incomplete.



We need to be able to provide a visual response to user input with minimal latency. In particular, on devices with touch input, content must track the finger exactly while panning, or the user experience is very poor. According to the UX team, 120ms is an acceptable latency between user input and response.

Context and surrounding architecture

The fundamental problem we are trying to solve with the Asynchronous Panning and Zooming (APZ) code is that of responsiveness. By default, web browsers operate in a “game loop” that looks like this:

while true:
    process input
    do computations
    repaint content
    display repainted content

In browsers the “do computation” step can be arbitrarily expensive because it can involve running event handlers in web content. Therefore, there can be an arbitrary delay between the input being received and the on-screen display getting updated.

Responsiveness is always good, and with touch-based interaction it is even more important than with mouse or keyboard input. In order to ensure responsiveness, we split the “game loop” model of the browser into a multithreaded variant which looks something like this:

Thread 1 (compositor thread)
while true:
    receive input
    send a copy of input to thread 2
    adjust painted content based on input
    display adjusted painted content

Thread 2 (main thread)
while true:
    receive input from thread 1
    do computations
    repaint content
    update the copy of painted content in thread 1

This multithreaded model is called off-main-thread compositing (OMTC), because the compositing (where the content is displayed on-screen) happens on a separate thread from the main thread. Note that this is a very very simplified model, but in this model the “adjust painted content based on input” is the primary function of the APZ code.

The “painted content” is stored on a set of “layers”, that are conceptually double-buffered. That is, when the main thread does its repaint, it paints into one set of layers (the “client” layers). The update that is sent to the compositor thread copies all the changes from the client layers into another set of layers that the compositor holds. These layers are called the “shadow” layers or the “compositor” layers. The compositor in theory can continuously composite these shadow layers to the screen while the main thread is busy doing other things and painting a new set of client layers.

The APZ code takes the input events that are coming in from the hardware and uses them to figure out what the user is trying to do (e.g. pan the page, zoom in). It then expresses this user intention in the form of translation and/or scale transformation matrices. These transformation matrices are applied to the shadow layers at composite time, so that what the user sees on-screen reflects what they are trying to do as closely as possible.

Technical overview

As per the heavily simplified model described above, the fundamental purpose of the APZ code is to take input events and produce transformation matrices. This section attempts to break that down and identify the different problems that make this task non-trivial.


The content area that is painted and stored in a shadow layer is called the “displayport”. The APZ code is responsible for determining how large the displayport should be. On the one hand, we want the displayport to be as large as possible. At the very least it needs to be larger than what is visible on-screen, because otherwise, as soon as the user pans, there will be some unpainted area of the page exposed. However, we cannot always set the displayport to be the entire page, because the page can be arbitrarily long and this would require an unbounded amount of memory to store. Therefore, a good displayport size is one that is larger than the visible area but not so large that it is a huge drain on memory. Because the displayport is usually smaller than the whole page, it is always possible for the user to scroll so fast that they end up in an area of the page outside the displayport. When this happens, they see unpainted content; this is referred to as “checkerboarding”, and we try to avoid it where possible.

There are many possible ways to determine what the displayport should be in order to balance the tradeoffs involved (i.e. having one that is too big is bad for memory usage, and having one that is too small results in excessive checkerboarding). Ideally, the displayport should cover exactly the area that we know the user will make visible. Although we cannot know this for sure, we can use heuristics based on current panning velocity and direction to ensure a reasonably-chosen displayport area. This calculation is done in the APZ code, and a new desired displayport is frequently sent to the main thread as the user is panning around.

Multiple layers

Consider, for example, a scrollable page that contains an iframe which itself is scrollable. The iframe can be scrolled independently of the top-level page, and we would like both the page and the iframe to scroll responsively. This means that we want independent asynchronous panning for both the top-level page and the iframe. In addition to iframes, elements that have the overflow:scroll CSS property set are also scrollable, and also end up on separate scrollable layers. In the general case, the layers are arranged in a tree structure, and so within the APZ code we have a matching tree of AsyncPanZoomController (APZC) objects, one for each scrollable layer. To manage this tree of APZC instances, we have a single APZCTreeManager object. Each APZC is relatively independent and handles the scrolling for its associated layer, but there are some cases in which they need to interact; these cases are described in the sections below.

Hit detection

Consider again the case where we have a scrollable page that contains an iframe which itself is scrollable. As described above, we will have two APZC instances - one for the page and one for the iframe. When the user puts their finger down on the screen and moves it, we need to do some sort of hit detection in order to determine whether their finger is on the iframe or on the top-level page. Based on where their finger lands, the appropriate APZC instance needs to handle the input. This hit detection is also done in the APZCTreeManager, as it has the necessary information about the sizes and positions of the layers. Currently this hit detection is not perfect, as it uses rects and does not account for things like rounded corners and opacity.

Also note that for some types of input (e.g. when the user puts two fingers down to do a pinch) we do not want the input to be “split” across two different APZC instances. In the case of a pinch, for example, we find a “common ancestor” APZC instance - one that is zoomable and contains all of the touch input points, and direct the input to that APZC instance.

Scroll Handoff

Consider yet again the case where we have a scrollable page that contains an iframe which itself is scrollable. Say the user scrolls the iframe so that it reaches the bottom. If the user continues panning on the iframe, the expectation is that the top-level page will start scrolling. However, as discussed in the section on hit detection, the APZC instance for the iframe is separate from the APZC instance for the top-level page. Thus, we need the two APZC instances to communicate in some way such that input events on the iframe result in scrolling on the top-level page. This behaviour is referred to as “scroll handoff” (or “fling handoff” in the case where analogous behaviour results from the scrolling momentum of the page after the user has lifted their finger).

Input event untransformation

The APZC architecture by definition results in two copies of a “scroll position” for each scrollable layer. There is the original copy on the main thread that is accessible to web content and the layout and painting code. And there is a second copy on the compositor side, which is updated asynchronously based on user input, and corresponds to what the user visually sees on the screen. Although these two copies may diverge temporarily, they are reconciled periodically. In particular, they diverge while the APZ code is performing an async pan or zoom action on behalf of the user, and are reconciled when the APZ code requests a repaint from the main thread.

Because of the way input events are stored, this has some unfortunate consequences. Input events are stored relative to the device screen - so if the user touches at the same physical spot on the device, the same input events will be delivered regardless of the content scroll position. When the main thread receives a touch event, it combines that with the content scroll position in order to figure out what DOM element the user touched. However, because we now have two different scroll positions, this process may not work perfectly. A concrete example follows:

Consider a device with screen size 600 pixels tall. On this device, a user is viewing a document that is 1000 pixels tall, and that is scrolled down by 200 pixels. That is, the vertical section of the document from 200px to 800px is visible. Now, if the user touches a point 100px from the top of the physical display, the hardware will generate a touch event with y=100. This will get sent to the main thread, which will add the scroll position (200) and get a document-relative touch event with y=300. This new y-value will be used in hit detection to figure out what the user touched. If the document had a absolute-positioned div at y=300, then that would receive the touch event.

Now let us add some async scrolling to this example. Say that the user additionally scrolls the document by another 10 pixels asynchronously (i.e. only on the compositor thread), and then does the same touch event. The same input event is generated by the hardware, and as before, the document will deliver the touch event to the div at y=300. However, visually, the document is scrolled by an additional 10 pixels so this outcome is wrong. What needs to happen is that the APZ code needs to intercept the touch event and account for the 10 pixels of asynchronous scroll. Therefore, the input event with y=100 gets converted to y=110 in the APZ code before being passed on to the main thread. The main thread then adds the scroll position it knows about and determines that the user touched at a document-relative position of y=310.

Analogous input event transformations need to be done for horizontal scrolling and zooming.

Content independently adjusting scrolling

As described above, there are two copies of the scroll position in the APZ architecture - one on the main thread and one on the compositor thread. Usually for architectures like this, there is a single “source of truth” value and the other value is simply a copy. However, in this case that is not easily possible to do. The reason is that both of these values can be legitimately modified. On the compositor side, the input events the user is triggering modify the scroll position, which is then propagated to the main thread. However, on the main thread, web content might be running Javascript code that programmatically sets the scroll position (via window.scrollTo, for example). Scroll changes driven from the main thread are just as legitimate and need to be propagated to the compositor thread, so that the visual display updates in response.

Because the cross-thread messaging is asynchronous, reconciling the two types of scroll changes is a tricky problem. Our design solves this using various flags and generation counters. The general heuristic we have is that content-driven scroll position changes (e.g. scrollTo from JS) are never lost. For instance, if the user is doing an async scroll with their finger and content does a scrollTo in the middle, then some of the async scroll would occur before the “jump” and the rest after the “jump”.

Content preventing default behaviour of input events

Another problem that we need to deal with is that web content is allowed to intercept touch events and prevent the “default behaviour” of scrolling. This ability is defined in web standards and is non-negotiable. Touch event listeners in web content are allowed call preventDefault() on the touchstart or first touchmove event for a touch point; doing this is supposed to “consume” the event and prevent touch-based panning. As we saw in a previous section, the input event needs to be untransformed by the APZ code before it can be delivered to content. But, because of the preventDefault problem, we cannot fully process the touch event in the APZ code until content has had a chance to handle it. Web browsers in general solve this problem by inserting a delay of up to 300ms before processing the input - that is, web content is allowed up to 300ms to process the event and call preventDefault on it. If web content takes longer than 300ms, or if it completes handling of the event without calling preventDefault, then the browser immediately starts processing the events.

The way the APZ implementation deals with this is that upon receiving a touch event, it immediately returns an untransformed version that can be dispatched to content. It also schedules a 400ms timeout (600ms on Android) during which content is allowed to prevent scrolling. There is an API that allows the main-thread event dispatching code to notify the APZ as to whether or not the default action should be prevented. If the APZ content response timeout expires, or if the main-thread event dispatching code notifies the APZ of the preventDefault status, then the APZ continues with the processing of the events (which may involve discarding the events).

The touch-action CSS property from the pointer-events spec is intended to allow eliminating this 400ms delay in many cases (although for backwards compatibility it will still be needed for a while). Note that even with touch-action implemented, there may be cases where the APZ code does not know the touch-action behaviour of the point the user touched. In such cases, the APZ code will still wait up to 400ms for the main thread to provide it with the touch-action behaviour information.

Technical details

This section describes various pieces of the APZ code, and goes into more specific detail on APIs and code than the previous sections. The primary purpose of this section is to help people who plan on making changes to the code, while also not going into so much detail that it needs to be updated with every patch.

Overall flow of input events

This section describes how input events flow through the APZ code.

  1. Input events arrive from the hardware/widget code into the APZ via APZCTreeManager::ReceiveInputEvent. The thread that invokes this is called the input thread, and may or may not be the same as the Gecko main thread.

  2. Conceptually the first thing that the APZCTreeManager does is to associate these events with “input blocks”. An input block is a set of events that share certain properties, and generally are intended to represent a single gesture. For example with touch events, all events following a touchstart up to but not including the next touchstart are in the same block. All of the events in a given block will go to the same APZC instance and will either all be processed or all be dropped.

  3. Using the first event in the input block, the APZCTreeManager does a hit-test to see which APZC it hits. This hit-test uses the event regions populated on the layers, which may be larger than the true hit area of the layer. If no APZC is hit, the events are discarded and we jump to step 6. Otherwise, the input block is tagged with the hit APZC as a tentative target and put into a global APZ input queue.

    1. If the input events landed outside the dispatch-to-content event region for the layer, any available events in the input block are processed. These may trigger behaviours like scrolling or tap gestures.

    2. If the input events landed inside the dispatch-to-content event region for the layer, the events are left in the queue and a 400ms timeout is initiated. If the timeout expires before step 9 is completed, the APZ assumes the input block was not cancelled and the tentative target is correct, and processes them as part of step 10.

  4. The call stack unwinds back to APZCTreeManager::ReceiveInputEvent, which does an in-place modification of the input event so that any async transforms are removed.

  5. The call stack unwinds back to the widget code that called ReceiveInputEvent. This code now has the event in the coordinate space Gecko is expecting, and so can dispatch it to the Gecko main thread.

  6. Gecko performs its own usual hit-testing and event dispatching for the event. As part of this, it records whether any touch listeners cancelled the input block by calling preventDefault(). It also activates inactive scrollframes that were hit by the input events.

  7. The call stack unwinds back to the widget code, which sends two notifications to the APZ code on the input thread. The first notification is via APZCTreeManager::ContentReceivedInputBlock, and informs the APZ whether the input block was cancelled. The second notification is via APZCTreeManager::SetTargetAPZC, and informs the APZ of the results of the Gecko hit-test during event dispatch. Note that Gecko may report that the input event did not hit any scrollable frame at all. The SetTargetAPZC notification happens only once per input block, while the ContentReceivedInputBlock notification may happen once per block, or multiple times per block, depending on the input type.

    1. If the events were processed as part of step 4(i), the notifications from step 8 are ignored and step 10 is skipped.

    2. If events were queued as part of step 4(ii), and steps 5-8 take less than 400ms, the arrival of both notifications from step 8 will mark the input block ready for processing.

    3. If events were queued as part of step 4(ii), but steps 5-8 take longer than 400ms, the notifications from step 8 will be ignored and step 10 will already have happened.

  8. If events were queued as part of step 4(ii) they are now either processed (if the input block was not cancelled and Gecko detected a scrollframe under the input event, or if the timeout expired) or dropped (all other cases). Note that the APZC that processes the events may be different at this step than the tentative target from step 3, depending on the SetTargetAPZC notification. Processing the events may trigger behaviours like scrolling or tap gestures.

If the CSS touch-action property is enabled, the above steps are modified as follows: * In step 4, the APZC also requires the allowed touch-action behaviours for the input event. This might have been determined as part of the hit-test in APZCTreeManager; if not, the events are queued. * In step 6, the widget code determines the content element at the point under the input element, and notifies the APZ code of the allowed touch-action behaviours. This notification is sent via a call to APZCTreeManager::SetAllowedTouchBehavior on the input thread. * In step 9(ii), the input block will only be marked ready for processing once all three notifications arrive.

Threading considerations

The bulk of the input processing in the APZ code happens on what we call “the input thread”. In practice the input thread could be the Gecko main thread, the compositor thread, or some other thread. There are obvious downsides to using the Gecko main thread - that is, “asynchronous” panning and zooming is not really asynchronous as input events can only be processed while Gecko is idle. In an e10s environment, using the Gecko main thread of the chrome process is acceptable, because the code running in that process is more controllable and well-behaved than arbitrary web content. Using the compositor thread as the input thread could work on some platforms, but may be inefficient on others. For example, on Android (Fennec) we receive input events from the system on a dedicated UI thread. We would have to redispatch the input events to the compositor thread if we wanted to the input thread to be the same as the compositor thread. This introduces a potential for higher latency, particularly if the compositor does any blocking operations - blocking SwapBuffers operations, for example. As a result, the APZ code itself does not assume that the input thread will be the same as the Gecko main thread or the compositor thread.

Active vs. inactive scrollframes

The number of scrollframes on a page is potentially unbounded. However, we do not want to create a separate layer for each scrollframe right away, as this would require large amounts of memory. Therefore, scrollframes as designated as either “active” or “inactive”. Active scrollframes are the ones that do have their contents put on a separate layer (or set of layers), and inactive ones do not.

Consider a page with a scrollframe that is initially inactive. When layout generates the layers for this page, the content of the scrollframe will be flattened into some other PaintedLayer (call it P). The layout code also adds the area (or bounding region in case of weird shapes) of the scrollframe to the dispatch-to-content region of P.

When the user starts interacting with that content, the hit-test in the APZ code finds the dispatch-to-content region of P. The input block therefore has a tentative target of P when it goes into step 4(ii) in the flow above. When gecko processes the input event, it must detect the inactive scrollframe and activate it, as part of step 7. Finally, the widget code sends the SetTargetAPZC notification in step 8 to notify the APZ that the input block should really apply to this new layer. The issue here is that the layer transaction containing the new layer must reach the compositor and APZ before the SetTargetAPZC notification. If this does not occur within the 400ms timeout, the APZ code will be unable to update the tentative target, and will continue to use P for that input block. Input blocks that start after the layer transaction will get correctly routed to the new layer as there will now be a layer and APZC instance for the active scrollframe.

This model implies that when the user initially attempts to scroll an inactive scrollframe, it may end up scrolling an ancestor scrollframe. (This is because in the absence of the SetTargetAPZC notification, the input events will get applied to the closest ancestor scrollframe’s APZC.) Only after the round-trip to the gecko thread is complete is there a layer for async scrolling to actually occur on the scrollframe itself. At that point the scrollframe will start receiving new input blocks and will scroll normally.

WebRender Integration

The APZ code was originally written to work with the “layers” graphics backend. Many of the concepts (and therefore variable/function names) stem from the integration with the layers backend. After the WebRender backend was added, the existing code evolved over time to integrate with that backend as well, resulting in a bit of a hodge-podge effect. With that cautionary note out of the way, there are three main pieces that need to be understood to grasp the integration between the APZ code and WebRender. These are detailed below.


The APZCTreeManager keeps as part of its internal state a tree of HitTestingTreeNode instances. This is referred to as the HitTestingTree. As the name implies, this was used for hit-testing purposes, so that APZ could determine which scrollframe a particular incoming input event would be targeting. Doing the hit-test requires access to a bunch of state, such as CSS transforms and clip rects, as well as ancillary data like event regions, which affect how APZ reacts to input events.

With the layers backend, all this information was provided by a layer tree update, and so the HitTestingTree was created to mirror the layer tree, allowing APZ access to that information from other threads. The structure of the tree was identical to the layer tree. But with WebRender, there is no “layer tree” per se, and instead we “fake it” by creating a HitTestingTree structure that is similar to what it would be like on the equivalent layer tree. But the bigger difference is that with WebRender, the HitTestingTree is not actually used for hit-testing at all; instead we get WebRender to do the hit-test for us, as it can do so using its own internal state and produce a more precise result.

Information stored in the HitTestingTree (e.g. CSS transforms) is still used by other pieces of APZ (e.g. some of the scrollbar manipulation code) so it is still needed, even with the WebRender backend. For this reason, and for consistency between the two backends, we try to populate as much information in the HitTestingTree that we can, even with the WebRender backend.

With the layers backend, the way the HitTestingTree is created is by walking the layer tree with a LayerMetricsWrapper class. This wraps a layer tree but also expands layers with multiple ScrollMetadata into multiple nodes. The equivalent in the WebRender world is the WebRenderScrollDataWrapper, which wraps a WebRenderScrollData object. The WebRenderScrollData object is roughly analogous to a layer tree, but is something that is constructed deliberately rather than being a natural output from the WebRender paint transaction (i.e. we create it explicitly for APZ’s consumption, rather than something that we would create anyway for WebRender’s consumption).

The WebRenderScrollData structure contains within it a tree of WebRenderLayerScrollData instances, which are analogous to individual layers in a layer tree. These instances contain various fields like CSS transforms, fixed/sticky position info, etc. that would normally be found on individual layers in the layer tree. This allows the code that builds the HitTestingTree to consume either a WebRenderScrollData or a layer tree in a more-or-less unified fashion.

Working backwards a bit more, the WebRenderLayerScrollData instances are created as we traverse the Gecko display list and build the WebRender display list. In the layers world, the code in FrameLayerBuilder was responsible for building the layer tree from the Gecko display list, but in the WebRender world, this happens primarily in WebRenderCommandBuilder. As of this writing, the architecture for this is that, as we walk the Gecko display list, we query it to see if it contains any information that APZ might need to know (e.g. CSS transforms) via a call to nsDisplayItem::UpdateScrollData(nullptr, nullptr). If this call returns true, we create a WebRenderLayerScrollData instance for the item, and populate it with the necessary information in WebRenderLayerScrollData::Initialize. We also create WebRenderLayerScrollData instances if we detect (via ASR changes) that we are now processing a Gecko display item that is in a different scrollframe than the previous item. This is equivalent to how FrameLayerBuilder will flatten items with different ASRs into different layers, so that it is cheap to scroll scrollframes in the compositor.

The main sources of complexity in this code come from:

  1. Ensuring the ScrollMetadata instances end on the proper WebRenderLayerScrollData instances (such that every path from a leaf WebRenderLayerScrollData node to the root has a consistent ordering of scrollframes without duplications).

  2. The deferred-transform optimization that is described in more detail at the declaration of StackingContextHelper::mDeferredTransformItem.


Since the HitTestingTree is not used for actual hit-testing purposes with the WebRender backend (see previous section), this section describes how hit-testing actually works with WebRender.

With both layers and WebRender, the Gecko display list contains display items (nsDisplayCompositorHitTestInfo) that store hit-testing state. These items implement the CreateWebRenderCommands method and generate a “hit-test item” into the WebRender display list. This is basically just a rectangle item in the WebRender display list that is no-op for painting purposes, but contains information that should be returned by the hit-test (specifically the hit info flags and the scrollId of the enclosing scrollframe). The hit-test item gets clipped and transformed in the same way that all the other items in the WebRender display list do, via clip chains and enclosing reference frame/stacking context items.

When WebRender needs to do a hit-test, it goes through its display list, taking into account the current clips and transforms, adjusted for the most recent async scroll/zoom, and determines which hit-test item(s) are under the target point, and returns those items. APZ can then take the frontmost item from that list (or skip over it if it happens to be inside a OOP subdocument that’s pointer-events:none) and use that as the hit target. It’s important to note that when APZ does hit-testing for the layers backend, it uses the most recent available async transforms, even if those transforms have not yet been composited. With WebRender, the hit-test uses the last transform provided by the SampleForWebRender API (see next section) which generally reflects the last composite, and doesn’t take into account further changes to the transforms that have occurred since then.

When debugging hit-test issues, it is often useful to apply the patches on bug 1656260, which introduce a guid on Gecko display items and propagate it all the way through to where APZ gets the hit-test result. This allows answering the question “which nsDisplayCompositorHitTestInfo was responsible for this hit-test result?” which is often a very good first step in solving the bug. From there, one can determine if there was some other display item in front that should have generated a nsDisplayCompositorHitTestInfo but didn’t, or if display item itself had incorrect information. The second patch on that bug further allows exposing hand-written debug info to the APZ code, so that the WR hit-testing mechanism itself can be more effectively debugged, in case there is a problem with the WR display items getting improperly transformed or clipped.


With both the layers and WebRender backend, the compositing step needs to read the latest async transforms from APZ in order to ensure scrollframes are rendered at the right position. In both cases, the API for this is exposed via the APZSampler class. The difference is that with the layers backend, the AsyncCompositionManager walks the layer tree and queries the transform components for each layer individually via the various getters on APZSampler. In contrast, with the WebRender backend, there is a single APZSampler::SampleForWebRender API that returns all the information needed for all the scrollframes, scrollthumbs, etc. Conceptually though, the functionality is pretty similar, because the compositor needs the same information from APZ regardless of which backend is in use.

Along with sampling the APZ transforms, the compositor also triggers APZ animations to advance to the next timestep (usually the next vsync). Again, with both the WebRender and layers backend, this happens just before reading the APZ transforms. The only difference is that with the layers backend, the AsyncCompositionManager invokes the APZSampler::AdvanceAnimations API directly, whereas with the WebRender backend this happens as part of the APZSampler::SampleForWebRender implementation.

Threading / Locking Overview


There are three threads relevant to APZ: the controller thread, the updater thread, and the sampler thread. This table lists which threads play these roles on each platform / configuration:

APZ Thread Name







controller thread

UI main

GPU main

UI main

GPU main

Java UI

Java UI

updater thread







sampler thread








There are also a number of locks used in APZ code:

Lock type

How many instances

APZ tree lock

one per APZCTreeManager

APZC map lock

one per APZCTreeManager

APZC instance lock

one per AsyncPanZoomController

APZ test lock

one per APZCTreeManager

Checkerboard event lock

one per AsyncPanZoomController

Thread / Lock Ordering

To avoid deadlocks, the threads and locks have a global ordering which must be respected.

Respecting the ordering means the following:

  • Let “A < B” denote that A occurs earlier than B in the ordering

  • Thread T may only acquire lock L, if T < L

  • A thread may only acquire lock L2 while holding lock L1, if L1 < L2

  • A thread may only block on a response from another thread T while holding a lock L, if L < T

The lock ordering is as follows:

  1. UI main

  2. GPU main (only if GPU enabled)

  3. Compositor thread

  4. SceneBuilder thread (only if WR enabled)

  5. APZ tree lock

  6. RenderBackend thread (only if WR enabled)

  7. APZC map lock

  8. APZC instance lock

  9. APZ test lock

  10. Checkerboard event lock

Example workflows

Here are some example APZ workflows. Observe how they all obey the global thread/lock ordering. Feel free to add others:

  • Input handling (in WR+GPU) case: UI main -> GPU main -> APZ tree lock -> RenderBackend thread

  • Sync messages in PCompositorBridge.ipdl: UI main thread -> Compositor thread

  • GetAPZTestData: Compositor thread -> SceneBuilder thread -> test lock

  • Scene swap: SceneBuilder thread -> APZ tree lock -> RenderBackend thread

  • Updating hit-testing tree: SceneBuilder thread -> APZ tree lock -> APZC instance lock

  • Updating APZC map: SceneBuilder thread -> APZ tree lock -> APZC map lock

  • Sampling and animation deferred tasks 1: RenderBackend thread -> APZC map lock -> APZC instance lock

  • Advancing animations: RenderBackend thread -> APZC instance lock


It looks like there are two deferred tasks that actually need the tree lock, AsyncPanZoomController::HandleSmoothScrollOverscroll and AsyncPanZoomController::HandleFlingOverscroll. We should be able to rewrite these to use the map lock instead of the tree lock. This will allow us to continue running the deferred tasks on the sampler thread rather than having to bounce them to another thread.