* Revert "Fix documentation build for window changes. (#21780)"

This reverts commit 931a0468.

* Revert "Migration to PlatformDispatcher and multi-window (#20496)"

This reverts commit 85b0031f.

This is a PR for converting the dart:ui code in the engine to use a multi-window API. The goal here is to convert from the window singleton to an API that has the concept of multiple windows. Also, I'm matching up the new PlatformDispatcher class to talk directly to the PlatformConfiguration class in the engine. I'm not attempting to actually enable creating multiple windows here, just migrate to an API that has a concept of multiple windows. The multi-window API in this PR currently only ever creates one window.

The design doc for this change is here.

The major changes in this PR:

Move the platfom-specific attributes out of Window, and into the new PlatformDispatcher class that holds all of the platform state, so that the platform code need only update the configuration on this class.
Create FlutterView, FlutterWindow, and SingletonFlutterWindow classes to separate out the concepts of a view (of which there may be multiple in a window), a window (of which there may be multiple on a screen, and they host views), and a window where there is only ever expected to be one (this hosts the entire API of the former Window class, and will eventually be the type of the window singleton).
Next step after this PR lands:

Remove the Window class entirely (it is replaced by SingletonFlutterWindow). Some minor changes in the Framework are needed to switch to using SingletonFlutterWindow directly first.

The Window class still exists in this PR, but will be removed as soon as the framework is converted to point to the SingletonFlutterWindow class instead. They share the same API, just have different names (Window is currently a subclass of SingletonFlutterWindow). The intention is that the Window name will be freed up to use as a widget class name in the framework for managing windows. The singleton called window will remain, and keep the same API it has now.

* Avoid sending a 0 DPR to framework

* check width and height as well

Cleans up header order/grouping for consistency: associated header, C/C++ system/standard library headers, library headers, platform-specific #includes.

Adds <cstring> where strlen, memcpy are being used: there are a bunch of places we use them transitively.

Applies linter-required cleanups. Disables linter on one file due to included RapidJson header. See https://github.com/flutter/flutter/issues/65676

This patch does not cover flutter/shell/platform/darwin. There's a separate, slightly more intensive cleanup for those in progress.

We currently use a mix of C standard includes (e.g. limits.h) and their
C++ variants (e.g. climits). This migrates to a consistent style for all
cases where the C++ variants are acceptable, but leaves the C
equivalents in place where they are required, such as in the embedder
API and other headers that may be used from C.

Reland #19396 with a fix for improper scale that was affecting internal tests

Tested: Ran all unittests, ran internal tests, and ran workstation on Fuchsia
BUG: 53062, 53063

This reverts commit 12a37478.

The fuchsia code around metrics and sizing was just sending this
information through a side-channel, when the engine already had the
information available. So, delete all of it to make future CLs simpler.

Additionally, the SceneUpdateContext has many unneccesary dependencies
re: metrics and PaintTasks. Break those to make future CLs simpler.

Tested: Ran all unittests and ran workstation on Fuchsia
BUG: 53062, 53063

This fixes all of the lint errors in lib/ui, except for a few (three, I think) where it would have changed the API, converting non-const references to pointers. For those, I just did NOLINT on the particular line instead of ignoring the whole file.

This is part of a larger effort to expose the difference between GrDirectContext,
which runs on the GPU thread and can directly perform operations like uploading
textures, and GrRecordingContext, which can only queue up work to be delivered
to the GrDirectContext later.

This change makes it so that we track all the motion events encountered by `FlutterView` and all of its subviews in the `MotionEventTracker` class, indexed by a unique `MotionEventId`. This identifier is then passed to the Flutter framework as seen in https://github.com/flutter/flutter/pull/60930. Once the gestures take part in gesture disambiguation and are sent back to the engine, we look-up the original motion event using the `MotionEventId` and dispatch it to the platform.

Bug: https://github.com/flutter/flutter/issues/58837

* Revert "Do not register UI-related native functions in secondary isolates (#6401)"

This reverts commit 69ae5694 as it doesn't work when root and secondary isolates run in the same isolate group.

* Confirm it is root isolate that makes UI native calls.

* Fix format, UIDartState reference from Fuchsia source

* No UI isolate check for fuchsia calls

* Fix typo. Remove redundant runtime calls

flutter/flutter#54022

The original issue is the _MotionEventsDispatcher depends on all down pointers to send out their own move events. If the third pointer has the movement of zero, it will get filter out by the PointerDataPacketConverter. This result with only two pointer move events been sent to RenderAndroidView, and they both get ignored because it is waiting for the third one.

This pr fixes it by always forward the move event

Broke a fake implementation of Accessibility features in the framework.
While this change is unlikely to break any users in the wild, this is a
breaking change by the standards set out in our documentation and
should follow the breaking change announcement process. Details can be
found at https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes.

The error in question:

    Missing concrete implementation of 'getter AccessibilityFeatures.onOffSwitchLabels' • packages/flutter_test/test/window_test.dart:252:7 • non_abstract_class_inherits_abstract_member

This reverts commit d12f2a60.

This reverts commit 6ea69a0d.

On top of the revert, it reverted a commit in the PR:
https://github.com/flutter/engine/pull/14024

This reverts commit ea67e5b0b930ebf552bc7dbd678a35ee6a129c39.

This reverts commit d117ac97.

* Remove erroneous ChildView opacity

* Wire frame metrics through contexts

* Maintain layer stack inside of SceneBuilder

* Remove EnsureSingleChild

* Centralize system-composite and elevation logic

* Wire up OpacityLayer to Scenic

* Moves pointer event sanitizing to engine

* fix comment format

* fix formatting

* addressing comment

* fix format

* fix format

* addressing comment

Since this is currently only meant to be used by the embedding internally, the setter in Objective-C is only exposed via the FlutterDartProject private class extension. Unit tests have been added to the shell_unittests harness.

Fixes https://github.com/flutter/flutter/issues/37641

This reverts commit fcc4ab32.

Fixes https://github.com/flutter/flutter/issues/41394 and other
related correctness issues.

TBR: @arbreng @jason-simmons @mehmetf 

This reverts commit 147d0c38.

On Fuchsia, add a build flag for compositing OpacityLayers using the system
compositor vs Skia, which exposes a fastpath for opacity via Scenic.
This will only work under certain circumstances, in particular nested
OpacityLayers will not render correctly!

On Fuchsia, add a build flag for compositing PhysicalShapeLayers using
the system compositor vs Skia. Set to off by default, which restores
performant shadows on Fuchsia.

Remove the opacity exposed from ChildView, as that was added mistakenly.

Finally, we centralize the logic for switching between the
system-composited and in-process-composited paths inside of
ContainerLayer. We also centralize the logic for computing elevation
there. This allows the removal of many OS_FUCHSIA-specific code-paths.

Test: Ran workstation on Fuchsia; benchmarked before and after
Bug: 23711
Bug: 24163

* Fix broken tests

* Pass Android Q system gesture inset information from insets.systemGestureInsets to Window.systemGestureInsets

Using it, a Flutter app can monitor missing frames in the release mode, and a custom Flutter runner (e.g., Fuchsia) can add a custom FrameRasterizedCallback.

Related issues:
https://github.com/flutter/flutter/issues/26154
https://github.com/flutter/flutter/issues/31444
https://github.com/flutter/flutter/issues/32447

Need review as soon as possible so we can merge this before the end of May to catch the milestone.

Tests added:
* NoNeedToReportTimingsByDefault
* NeedsReportTimingsIsSetWithCallback
* ReportTimingsIsCalled
* FrameRasterizedCallbackIsCalled
* FrameTimingSetsAndGetsProperly
* onReportTimings preserves callback zone
* FrameTiming.toString has the correct format

This will need a manual engine roll as the TestWindow defined in the framework needs to implement onReportTimings.

* Synthesize a buttons = kPrimaryButton for events of down and move
* Add PointerEventButtons

This is the first step in making Flutter aware of and responsive to Z
bound overflow. On its own this patch shouldn't result in any changes in
behavior. This will need to be followed up with a patch in Fuchsia's
Flutter runner to set the Z bounds after this lands, and another patch
in the engine actually implementing the desired overflow behavior.

This Z bound info is routed through the engine itself to make sure the
bounds in flow are truly consistent from the Fuchsia runner. However
these values should eventually be exposed to the framework as well.