`RenderSemanticsGestureHandler` is no longer a semantics boundary, which allows us to correctly mark disabled buttons as disabled without having their semantics size and semantics node id change unexpectedly.

Fixes https://github.com/flutter/flutter/issues/12589.
Fixes https://github.com/flutter/flutter/issues/11991.
See also https://github.com/flutter/flutter/issues/11993.

This change also required some refactoring to how we deal with `twoPaneSemantics` scrolling as it previously relied on `RenderSemanticsGestureHandler` being a semantics boundary. This should also make the underlying logic easier to understand.

In addition, the following minor changes are included in this PR:
* Removal of orphaned and unused `SemanticsConfiguration.isMergingDescendantsIntoOneNode`.
* Logic optimizations for `markNeedsSemanticsUpdate` .
* Fix for edge case where `MergeSemantics` failed to merge semantics.
* Use of emojis to better indicate leaf merging in the printed semantics tree.
* Better assert message for adding invisible child semantics nodes.
* Make some semantics tests robuster by not relying on creation order of SemanticsNode ids across test boundaries.

Fixes https://github.com/flutter/flutter/issues/13943.

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.

* fix miscellaneous typos in dartdocs

* moar typos

* Oct 12 10:12am

* implicit_semantics_test.dart passes

* refactoring

* works in nice

* minor rename

* more doc comments

* to be explicit check better

* fix test

* ++

* ++

* semantics_9_test (BlockSemantics) and implicit_semantics_test are passing

* doc updates

* tiny refactor

* fix static errors in tests

* fix gesture detector

* ++

* ++

* geometry

* ++

* remove noGeometry

* revert test

* +

* all tests but scrolling/clipping pass

* clipping works

* scrolling halfway

* sliver tests pass

* ALL TESTS PASS

* SemanticsNode changed

* docs and tiny fixes

* card test

* more doc comments

* remove missed print

* more tests

* make test pass on Linux

* remove changes to intellij proj file

* review comments

* getOffsetToReveal deals with pinned slivers

If a Sliver can potentially be pinned at the edge of a viewport, getOffsetToReveal will take that into account and scroll further up/down to ensure that the object to reveal doesn't end up covered by a pinned sliver.

This is important for accessibility scrolling with app bars.

Since how much a pinned sliver is covering is dynamic (it can change with scroll offset, etc), getOffsetToReveal deals with the worst case and tries to ensure that the object to uncover is visible when the pinned slivers are at their max pinned extent.

* name fixes

* review feedback

* typos

* renaming

* fix analyzer

* fix test

* analyzer fixes

Fixes #11855

* ensureVisible works with viewports that have a center child

* review feedback

* Exclude SemanticNodes that are fully covered by an overlapping sliver

* simplification

* formatting

* nits

* add test for center widget

* Semantic hit test order for Slivers

* ++

* fix double space

* refactoring

* fix test

* comment fix

* tests

* remove accidental debugDumpSemanticsTree

* review comments

* fix button

* refactor to ensureAction + some related doc fixes

* Update docs for markNeedsSemanticsUpdate

* rewording

* rewording

* ensureAction test

* ensureAction test

* ensureAction test

* more tests

* refactor to allways reset node

* tiny fixes

* more test

* doc fixes

* one more test

* review comments

* refactor to assembleSemanticNode

* ++

* cleanup

* fix test

* add note

* review comments

* review feedback

* import fix

* another import fix

* refactor to ensure tag

* tests, tests, tests

* analyzer fixes

* review comments

Add Diagnosticable base class and documentation

Refactor RenderObject.toStringDeep and Widget.toStringDeep code to use DiagnosticsNode.

* a11y: implement new SemanticsAction "showOnScreen" (v2)

This action is triggered when the user swipes (in accessibility mode) to the last visible item of a scrollable list to bring that item fully on screen.

This requires engine rolled to flutter/engine#3856.

I am in the process of adding tests, but I'd like to get early feedback to see if this approach is OK.

* fix null check

* review comments

* review comments

* Add test

* fix analyzer warning

Fixes https://github.com/flutter/flutter/issues/10547

This prevents some of our mixins from being subclassed.

Also, move mixins to using 'extends' instead of 'implements' for
future compatibility with Dart changes.

Also, rename a class that had Mixin in the name but was not a mixin.

* Manually fix every use of Point.x and Point.y

Some of these were moved to dx/dy, but not all.

* Manually convert uses of the old gradient API

* Remove old reference to Point.

* Mechanical changes

I applied the following at the root of the Flutter repository:

git ls-files -z | xargs -0 sed -i 's/\bPoint[.]origin\b/Offset.zero/g'
git ls-files -z | xargs -0 sed -i 's/\bPoint[.]lerp\b/Offset.lerp/g'
git ls-files -z | xargs -0 sed -i 's/\bnew Point\b/new Offset/g'
git ls-files -z | xargs -0 sed -i 's/\bconst Point\b/const Offset/g'
git ls-files -z | xargs -0 sed -i 's/\bstatic Point /static Offset /g'
git ls-files -z | xargs -0 sed -i 's/\bfinal Point /final Offset /g'
git ls-files -z | xargs -0 sed -i 's/^\( *\)Point /\1Offset /g'
git ls-files -z | xargs -0 sed -i 's/ui[.]Point\b/ui.Offset/g'
git ls-files -z | xargs -0 sed -i 's/(Point\b/(Offset/g'
git ls-files -z | xargs -0 sed -i 's/\([[{,]\) Point\b/\1 Offset/g'
git ls-files -z | xargs -0 sed -i 's/@required Point\b/@required Offset/g'
git ls-files -z | xargs -0 sed -i 's/<Point>/<Offset>/g'
git ls-files -z | xargs -0 sed -i 's/[.]toOffset()//g'
git ls-files -z | xargs -0 sed -i 's/[.]toPoint()//g'
git ls-files -z | xargs -0 sed -i 's/\bshow Point, /show /g'
git ls-files -z | xargs -0 sed -i 's/\bshow Point;/show Offset;/g'

* Mechanical changes - dartdocs

I applied the following at the root of the Flutter repository:

git ls-files -z | xargs -0 sed -i 's/\ba \[Point\]/an [Offset]/g'
git ls-files -z | xargs -0 sed -i 's/\[Point\]/[Offset]/g'

* Further improvements and a test

* Fix minor errors from rebasing...

* Roll engine

Mostly related to semantics and scrolling.

This patch covers multi-box adaptor and the viewports.

Covers lib/ in package:flutter.

With slivers, the app bar is part of the scroll view. Naively, the overscroll
appears at the beginning of the scroll view, but that's not the desired
behavior. Instead, the app bar should remained pinned to the top of the scroll
view and the overscroll should appear below the app bar, which is what this
patch does.

Fixes #8228

Makes RenderViewport.layoutOneSide more readable.

Rather than generating a cryptic assert, we now generate a more useful error
message.

Fixes #8296

It doesn't need to be in the same file as RenderSliver.

All the clients have migrated to Scrollable2.

- ScrollableList
 - ScrollableLazyList
 - LazyBlock
 - MaterialList

Clients should use ListView instead.

LayoutCallback passes constraints to the callback, but the constraints
object has a different type for different subclasses. This lets you
call invokeLayoutCallback() with a specific type to verify that
everything is working as expected.

Other changes:

Slightly improve the error reporting in RenderObject.

Allow toStringShallow on RenderObject to have its separator configured.

Fundamentally the core problem was that we were not saying how wide a
date picker should be. It should be 330 pixels, if I'm measuring the
spec's mocks correctly.

We now have an EdgeInsets.along function that projects the padding onto the
given Axis rather than repeating this function in several places.