This allows us to propagate flags from the container query styles all the
way to the computed style of the element.
The flag for viewport units in container queries has to be different
because it requires rematching, see comments.
Depends on D159851
Differential Revision: https://phabricator.services.mozilla.com/D159852
Parsing is behind a config value `layout.css.has-selectors.enabled`. This
change does not support p:has(> a) combinators, but will handle them
gracefully, just not matching on them.
Differential Revision: https://phabricator.services.mozilla.com/D149515
Add a dom/base/rust crate called just "dom" where we can share these.
Most of the changes are automatic:
s/mozilla::EventStates/mozilla::dom::ElementState/
s/EventStates/ElementState/
s/NS_EVENT_STATE_/ElementState::/
s/NS_DOCUMENT_STATE_/DocumentState::/
And so on. This requires a new cbindgen version to avoid ugly casts for
large shifts.
Differential Revision: https://phabricator.services.mozilla.com/D148537
They are just convenience for :root[lwthemetextcolor="light"] (and dark,
respectively), but they generally shouldn't be used for dark mode
theming. In the past it was the only way to do it but now we have
prefers-color-scheme.
While at it, change lwthemetextcolor to be "lwtheme-brighttext" for
consistency with similar code we have for popups etc, and move it to
_setDarkModeAttributes.
While at it, remove layout.css.moz-lwtheme.content.enabled (which is
false always, we unshipped these from content successfully).
Differential Revision: https://phabricator.services.mozilla.com/D141593
* style: Simplify selector flag setting now that flag setting is atomic
These bits are write-only, actually, and we don't even need to read
them.
Differential Revision: https://phabricator.services.mozilla.com/D141888
* Further changes required by Servo
* style: Support media queries for dynamic-range and video-dynamic-range
This is a stub that only matches "standard" for all platforms.
Differential Revision: https://phabricator.services.mozilla.com/D141053
* style: Remove :-moz-lwtheme-{brighttext,darktext}
They are just convenience for :root[lwthemetextcolor="light"] (and dark,
respectively), but they generally shouldn't be used for dark mode
theming. In the past it was the only way to do it but now we have
prefers-color-scheme.
While at it, change lwthemetextcolor to be "lwtheme-brighttext" for
consistency with similar code we have for popups etc, and move it to
_setDarkModeAttributes.
While at it, remove layout.css.moz-lwtheme.content.enabled (which is
false always, we unshipped these from content successfully).
Differential Revision: https://phabricator.services.mozilla.com/D141593
* style: layout.css.moz-locale-dir.content.enabled
We successfully removed these from content in bug 1740230 (Firefox 96).
Differential Revision: https://phabricator.services.mozilla.com/D141727
* style: Really honor background-color: transparent in HCM even for form controls
I forgot we were doing this "revert-or-initial" shenanigans (which is needed
for stuff like link colors to be honored), so we need to early-return.
Use a more explicit test rather than a reftest for this.
Differential Revision: https://phabricator.services.mozilla.com/D142063
* style: Ignore unchanged property for scroll-linked effect detector
I think this is cleaner.
Differential Revision: https://phabricator.services.mozilla.com/D141737
* style: Allow to derive Parse/ToCss/SpecifiedValueInfo on bitflags
We keep getting this pattern of properties that have a set of joint and
disjoint flags, and copy-pasting or writing the same parsing and
serialization code in slightly different ways.
container-type is one such type, and I think we should have a single way
of dealing with this, thus implement deriving for various traits for
bitflags, with an attribute that says which flags are single vs mixed.
See docs and properties I ported. The remaining ones I left TODOs with,
they are a bit trickier but can be ported with some care.
Differential Revision: https://phabricator.services.mozilla.com/D142418
* Further changes required by Servo
* style: Implement parsing / serialization for container{,-type,-name} CSS properties
Two noteworthy details that may seem random otherwise:
* Moving values around in nsStyleDisplay is needed so that the struct
remains under the size limit that we have to avoid jumping allocator
buckets.
* All the test expectation churn is because tests depend on
`container-type: size` parsing to run, and now they run. Tests for
the relevant bits I implemented are passing, with the only exception
of some `container-name-computed.html` failures which are
https://github.com/w3c/csswg-drafts/issues/7181. Safari agrees with
us there.
Other notes when looking at the spec and seeing how it matches the
implementation:
* `container` syntax doesn't match spec, but matches tests and sanity:
https://github.com/w3c/csswg-drafts/issues/7180
* `container-type` syntax doesn't _quite_ match spec, but matches tests
and I think it's a spec bug since the definition for the missing
keyword is gone:
https://github.com/w3c/csswg-drafts/issues/7179
Differential Revision: https://phabricator.services.mozilla.com/D142419
* style: Remove assert that doesn't hold for text-decorations because of presentation hints
MANUAL PUSH: Orange fix CLOSED TREE
* style: Migrate `<th>` `text-align` behaviour from presentation hint to UA CSS
Differential Revision: https://phabricator.services.mozilla.com/D142494
* style: Deduplicate TokenList values faster
Remember whether we have already de-duplicated them once and avoid doing
that again.
This is an alternative approach that doesn't add overhead to attribute
setting in the general case.
Differential Revision: https://phabricator.services.mozilla.com/D142813
* style: Inherit used color-scheme from embedder <browser> elements
This allows popups and sidebars to use the chrome preferred
color-scheme.
This moves the responsibility of setting the content-preferred color
scheme to the appropriate browsers to the front-end (via tabs.css).
We still return the PreferredColorSchemeForContent() when there's no
pres context (e.g., for display:none in-process iframes). We could
potentially move a bunch of the pres-context data to the document
instead, but that should be acceptable IMO as for general web content
there's no behavior change in any case.
Differential Revision: https://phabricator.services.mozilla.com/D142578
* Further changes required by Servo
* style: Add basic @container rule parsing and boilerplate
For now parse a MediaFeatureCondition. That needs being made more
specific, but that is probably worth its own patch.
Differential Revision: https://phabricator.services.mozilla.com/D143192
* Further changes required by Servo
* style: Tweak cascade priority to split writing-mode and font properties
This makes the worst case for cascade performance slightly more
expensive (4 rather than three declaration walks), but my hope is that
it will make the average case faster, since the best case is now just
two walks instead of three, and writing mode properties are somewhat
rare.
This needs a test, but needs to wait until the writing-mode dependent
viewport units land (will wait to land with a test).
Differential Revision: https://phabricator.services.mozilla.com/D143261
* style: Implement new {small,large,dynamic} viewport units
Differential Revision: https://phabricator.services.mozilla.com/D143252
* Further changes required by Servo
* style: Implement new *vi and *vb units
Differential Revision: https://phabricator.services.mozilla.com/D143253
* style: Implement prefers-contrast: custom and let prefers-contrast ride the trains
Differential Revision: https://phabricator.services.mozilla.com/D143198
* style: Join servo style threads during shutdown
I was unable to change the BLOOM_KEY field to no longer be leaked, as the TLS
is also accessed on the main thread, which is not exited before the leak
checker shuts down.
Differential Revision: https://phabricator.services.mozilla.com/D143529
* Further changes required by Servo
* style: Fix visited handling after bug 1763750
Before bug 1763750, we unconditionally called compute_writing_mode,
which got the writing mode from the cascade mode for visited styles.
However after that bug we only do that if we apply any
writing-mode-related property.
We could just call compute_writing_mode unconditionally, but instead it
seems better to skip all that work for visited cascade and reuse the
mechanism introduced in that bug to only apply the visited-dependent
longhands.
We assert that all visited-dependent longhands are "late" longhands, so
as to also avoid applying the font group and such.
Differential Revision: https://phabricator.services.mozilla.com/D143490
* style: Clean-up viewport unit resolution a bit
I should've caught this when reviewing the new viewport units but alas :-)
Differential Revision: https://phabricator.services.mozilla.com/D143856
* style: Implement `contain: inline-size`
Differential Revision: https://phabricator.services.mozilla.com/D143501
* style: Simplify media query evaluation code a bit
This patch:
* Removes generic <ident> support for media features. These were used
for some privileged media features but are no longer used.
* Simplifies media feature getters by shifting the responsibility of
dealing with RangeOrOperator to the caller. This makes it easier to
implement container-query / mediaqueries-4 syntax, and also cleans up
the code a bunch.
There should be no change in behavior.
Differential Revision: https://phabricator.services.mozilla.com/D144051
* Further changes required by Servo
* style: Move transitions and animations to nsStyleUIReset
This mostly just moves code around, to minimize potential behavior
changes. There are some cleanups that we should try to do long term
(this "have an array with n different counts" is pretty weird).
But for now this should unblock people.
The destination struct (nsStyleUIReset) was chosen mainly because it's
small and non-inherited, and it doesn't seem like a worse place than
nsStyleDisplay.
Differential Revision: https://phabricator.services.mozilla.com/D144183
* Further changes required by Servo
* style: Make media feature evaluation take a computed::Context
This has no behavior change right now, but will simplify sharing code
with container queries.
Container queries will have container information in the
computed::Context (this is necessary anyways for container-based units),
so this avoids having to have different code for media and container
queries.
Differential Revision: https://phabricator.services.mozilla.com/D144152
* Further changes required by Servo
* style: Add scroll() to animation-timeline for style system
scroll() is defined in the spec proposal, and there is a temporary spec:
https://drafts.csswg.org/scroll-animations-1/rewrite#scroll-notation.
The spec is still under development, so we don't drop the orignal
scroll-timeline at rule. Instead, we add a new scroll() notation to
animation-timeline, and support both syntax for now.
Differential Revision: https://phabricator.services.mozilla.com/D143417
* style: Tweak contain bitflag definition order to avoid static constructors
This has no behavior change otherwise. The STRICT definition depended on
SIZE, which was defined later. That's fine in Rust, but in C++ it causes
the initialization to be dynamic because it doesn't have the definition
of SIZE yet (ugh).
This is the fix for the regression, though the following patch turns on
constexpr support in cbindgen, which would've caught this at build-time,
and guarantees that we don't have extra static constructors.
Differential Revision: https://phabricator.services.mozilla.com/D144316
* style: Move some of the media query code to a more generic queries module
No behavior change, just moving and renaming files.
The code in the "queries" module will be shared between @media and
@container.
@media has some other code that container queries doesn't need like
MediaList / MediaType / etc. That remains in the media_queries module.
Differential Revision: https://phabricator.services.mozilla.com/D144435
* Further changes required by Servo
* style: cleanup animation-name
Make the representation the same between Gecko and Servo code. This will
enable further clean-ups in the future.
Make serialization be correct, serializing as identifier unless it's an
invalid one (in which case we serialize as a string).
This changes our stringification behavior in the specified style, but
now it will match the computed style and be more correct over-all.
Differential Revision: https://phabricator.services.mozilla.com/D144473
* Further changes required by Servo
* style: Add support for parsing container-query-specific features
There are some mediaqueries-5 features that we still don't support and
explain the remaining failures in at-container-{parsing,serialization}.
Differential Revision: https://phabricator.services.mozilla.com/D144446
* Further changes required by Servo
* style: Introduce Optional<T> to represent optional values in the style system
cross-fade() was kinda doing this in its own way with PercentOrNone, but
since now we have more use-cases for this we should probably make this a
slightly more general solution.
I added some convenience APIs, but they're unused as of this patch so
let me know if you want them gone.
Differential Revision: https://phabricator.services.mozilla.com/D144831
* style: Fix insertRule with layer statements before imports
We need to do a bit more nuanced check because @layer statements might
go before imports.
Differential Revision: https://phabricator.services.mozilla.com/D144996
* style: Factor out parsing the query feature name
No behavior change.
Differential Revision: https://phabricator.services.mozilla.com/D145229
* style: Refactor media feature expression representation in preparation to support multi-range syntax
No behavior change.
Depends on D145229
Differential Revision: https://phabricator.services.mozilla.com/D145230
* style: Implement media feature expression multi-range syntax
Differential Revision: https://phabricator.services.mozilla.com/D145231
* style: Remove proton places tooltip code
There's nobody working on it, and tooltips should hopefully be nice
enough after recent changes (bug 1765423).
Having it enabled causes artifacts like bug 1767815 comment 3. We can
always rescue this from hg history if needed.
Differential Revision: https://phabricator.services.mozilla.com/D145621
* style: Simplify selector flags setup even more
In my investigation for bug 1766439, I am digging into why selector
matching regressed.
It doesn't help that the selector-matching code is instantiated a
gazillion times (so there's a ton of copies of the relevant functions).
This was needed in the past because we had different ways of setting the
selector flags on elements, but I unified that recently and now we only
need to either set them or not. That is the kind of thing that
MatchingContext is really good for, so pass that instead on
MatchingContext creation.
Differential Revision: https://phabricator.services.mozilla.com/D145428
* Further changes required by Servo
* style: Track @container condition id in style rules
Much like we track layer rules. Consolidate that "containing rule state
we pass down while building the cascade data" in a single struct that we
can easily restore.
For now, do nothing with it. I want to land this patch separately
because it touches the Rule struct and CascadeData rebuilds, which both
are performance sensitive.
Its layout shouldn't change because I also changed LayerId to be a u16
(this shouldn't matter in practice, since LayerOrder is already a u16).
Differential Revision: https://phabricator.services.mozilla.com/D145243
* Further changes required by Servo
* style: Fix layer statement rules with multiple layer names
MANUAL PUSH: Trivial orange fix CLOSED TREE.
* style: Implement piecewise linear function
Differential Revision: https://phabricator.services.mozilla.com/D145256
* style: Convert specified value tests to compile-time tests
These were written at a time where std::mem::size_of wasn't a `const fn` in
Rust.
Now that it is, we can make these tests live in the style crate, and the build
not to compile if they fail.
Differential Revision: https://phabricator.services.mozilla.com/D146103
* Further changes required by Servo
* style: Move size of tests to compile-time tests in the style crate
Same reasoning as the previous commit.
Differential Revision: https://phabricator.services.mozilla.com/D146104
* Further changes required by Servo
* style: Lint and 32-bit build fix.
MANUAL PUSH: Bustage fix CLOSED TREE
* style: Implement 'update' media feature
Differential Revision: https://phabricator.services.mozilla.com/D146338
* style: Clean up unused -moz-window-shadow values
After bug 1768278 and bug 1767815 there's no more uses of the cliprounded value
in the tree (also it causes artifacts on HiDPI screens so we probably don't
want new usages).
The "sheet" value is unused, and the other values other than "default" and
"none" are only derived from "default", so they don't need to be exposed in the
style system.
Differential Revision: https://phabricator.services.mozilla.com/D145821
* style: More container queries plumbing
Provide container information in computed::Context and use it to resolve
the container queries.
This still fails a lot of tests because we are not ensuring that layout
is up-to-date when we style the container descendants, but that's
expected.
Differential Revision: https://phabricator.services.mozilla.com/D146478
* Further changes required by Servo
* style: Ensure options in listbox selects are not stacking contexts by default
We could have a different property or something but this seems
reasonable as well probably.
Differential Revision: https://phabricator.services.mozilla.com/D146994
* style: Implement overflow-clip-margin: <length>
Differential Revision: https://phabricator.services.mozilla.com/D146432
* style: Change order of container shorthand
Since the initial value of container-type is an open issue [1],
I'm leaving that as-is for now.
[1] https://github.com/w3c/csswg-drafts/issues/7202
Differential Revision: https://phabricator.services.mozilla.com/D147338
* style: Make modal dialog code more generic, and make it apply to fullscreen too behind a pref
For now, don't turn it on by default yet, because I want to wait for
more discussion in https://github.com/w3c/csswg-drafts/issues/6965 and
so on. But I think the code is simple enough to land this.
Differential Revision: https://phabricator.services.mozilla.com/D147295
* style: Cache computed styles objects display: none subtrees
This reuses our existing undisplayed style generation, but in a
per-document rather than per-nsComputedDOMStyle object, which means that
we can avoid re-resolving styles of elements in display: none subtrees
much more often.
This brings the test-case in the bug to par with other browsers or
better, and is much simpler than the initial approach I tried back in
the day.
Differential Revision: https://phabricator.services.mozilla.com/D147547
* Further changes required by Servo
* style: Parse scroll-snap-stop style and propagate it to APZ side
Depends on D146147
Differential Revision: https://phabricator.services.mozilla.com/D145850
* style: Update color-mix() syntax to match the current spec
Test expectation updates for this in the latest patch of the bug.
Differential Revision: https://phabricator.services.mozilla.com/D147002
* Further changes required by Servo
* style: Make the color interpolation code more generic
It's really piece-wise premultiplied interpolation, with a special-case
for hue, so centralize the implementation.
Differential Revision: https://phabricator.services.mozilla.com/D147003
* style: Implement more color-mix() color-spaces
We had code to convert between these and the latest draft supports them so...
Differential Revision: https://phabricator.services.mozilla.com/D147004
* style: Fix color-mix() percentage normalization
Differential Revision: https://phabricator.services.mozilla.com/D147005
* style: Fix hue adjustment to match the spec
The value to sum is tau, not pi. This was caught by some tests, see
https://drafts.csswg.org/css-color/#shorter
Differential Revision: https://phabricator.services.mozilla.com/D147006
* style: Do hue interpolations in degrees rather than radians. r=barret
This gives us a bit more precision.
Differential Revision: https://phabricator.services.mozilla.com/D147007
* style: Improve Percentage -> LengthPercentage conversion
This doesn't change behavior because we only use them for images that
have no clamping.
Depends on D147008
Differential Revision: https://phabricator.services.mozilla.com/D147511
* style: Remove some dead vibrancy code
Drive-by cleanup.
Differential Revision: https://phabricator.services.mozilla.com/D147698
* style: Fix warnings about whitelist/blocklist functions being deprecated in bindgen 0.59
Differential Revision: https://phabricator.services.mozilla.com/D147695
* style: Update style to arrayvec 0.7
Differential Revision: https://phabricator.services.mozilla.com/D147476
* style: Update style to uluru 3.0
Differential Revision: https://phabricator.services.mozilla.com/D147477
* style: Remove -moz-scrollbar-thumb-proportional
It unconditionally matches on all platforms, so it's not returning any useful information.
Depends on D147689
Differential Revision: https://phabricator.services.mozilla.com/D147690
* style: Use ColorMix for interpolated colors in the computed style rather than ComplexColorRatios
This among other things preserves the right color-space when
interpolating currentColor.
Differential Revision: https://phabricator.services.mozilla.com/D147512
* Further changes required by Servo
* style: Add an input-region-margin to widgets, and implement it on Linux
Recompute the input region when resizing the widget and so on, and use
it to check for rollups.
Depends on D148211
Differential Revision: https://phabricator.services.mozilla.com/D148222
* Avoid complaints from ./mach test-tidy
* Update test expectations
---------
Co-authored-by: Emilio Cobos Álvarez <emilio@crisal.io>
Co-authored-by: Brad Werth <bwerth@mozilla.com>
Co-authored-by: David Shin <dshin@mozilla.com>
Co-authored-by: Hiroyuki Ikezoe <hikezoe.birchill@mozilla.com>
Co-authored-by: Nika Layzell <nika@thelayzells.com>
Co-authored-by: Boris Chiou <boris.chiou@gmail.com>
Co-authored-by: Autumn on Tape <autumn@cyfox.net>
Co-authored-by: Mike Hommey <mh+mozilla@glandium.org>
Behind a pref for now. Given these selectors do nothing on non-chrome
documents (they just don't match) it seems worth trying.
A cursory search seems to indicate they're not used for UA detection or
something like that (or at least I haven't found such an usage).
Differential Revision: https://phabricator.services.mozilla.com/D130736
Use the same document state mechanism we have for :moz-locale-dir. Also,
simplify the setup of the later to be the same as :dir(), allowing the
matching code to be less repetitive.
This should fix some flakiness in chrome mochitests, but we have no existing
tests for these pseudo-classes more generally and since they're just
chrome-only I'm not super-excited about adding more.
Differential Revision: https://phabricator.services.mozilla.com/D130735
This I noticed while working on the following patches. Shouldn't have
any behavior change: the behavior does in fact match the element state
flag semantics correctly if we do this. We did split the dir flags into
two element bits a while ago.
:not(:dir()) still behaves correctly of course, and we have tests for that.
Differential Revision: https://phabricator.services.mozilla.com/D130734
See the discussion here: https://twitter.com/Rich_Harris/status/1433153204678799365
This should make attribute selectors roughly as fast as class selectors.
I think it's worth trying and see if perf bots complain on
micro-benchmarks and stylebench and such.
I made attributes more specific than local names, but less specific than
classes, which I think makes sense. When doing something like
foo[data-bar], filtering by data-bar seems likely to yield less elements
than filtering by foo.
While at it, remove the bloom filter pref since we shipped it in
bug 1704551 for 87 and we haven't heard complaints.
Differential Revision: https://phabricator.services.mozilla.com/D124383
Safari does this. This reduces the runtime in the example linked from
comment 0 quite a lot (40ms on a local opt build, from ~130ms on a
release nightly build).
I added a pref because there's a slight chance of performance
regressions on pages that do not use attribute selectors, as we're now
doing more unconditional work per element (adding the attributes to the
bloom filter). But the trade-off should be worth it, I think.
Differential Revision: https://phabricator.services.mozilla.com/D111689
And alias :-moz-ui-valid and :-moz-ui-invalid to them.
There are CSSWG resolutions for these for quite a while, and spec for
user-invalid.
Differential Revision: https://phabricator.services.mozilla.com/D105966
No other browser supports anything like this and we don't even have
internal users. Only uses of this I've found on the wild were just
resetting the box shadow internal styling we added in bug 582277 (and
since removed in bug 600151).
Differential Revision: https://phabricator.services.mozilla.com/D105955
This shipped in 85, we can remove the feature flag now. Keep
:-moz-focusring as an alias to :focus-visible at parse time.
Differential Revision: https://phabricator.services.mozilla.com/D103752
Adjust is-where-parsing.html to work with both the new and old behavior,
and add a test for the new behavior.
Depends on D90049
Differential Revision: https://phabricator.services.mozilla.com/D90050
This is strictly better and more flexible, but can change specificity so
have a pref in case it causes trouble. I doubt it will though, the
specificity rules of :is() make more sense, and my gut feeling is that
:-moz-any is not very used on the wild.
Make it early-beta-or-earlier for now to minimize risk, once this is on
nightly for a bit we can enable it everywhere.
Differential Revision: https://phabricator.services.mozilla.com/D86696
This should help out quite a bit with uBO, which has lots of very
general attribute selectors. We invalidate per attribute name rather
than using a SelectorMap, which prevents matching for attribute
selectors that can't have changed.
The idea is that this should be generally cheaper, though there are
cases where this would be a slight pesimization. For example, if there's
an attribute selector like:
my-specific-element[my-attribute] { /* ... */ }
And you change `my-attribute` in an element that isn't a
`my-specific-element`, before that the SelectorMap would've prevented us
from selector-matching completely. Now we'd still run selector-matching
for that (though the matching would be pretty cheap).
However I think this should speed up things generally, let's see what
the perf tests think before landing this though.
Differential Revision: https://phabricator.services.mozilla.com/D76825
This implements the easy / straight-forward parts of the :where / :is
selectors.
The biggest missing piece is to handle properly invalidation when there
are combinators present inside the :where. That's the hard part of this,
actually.
But this is probably worth landing in the interim. This fixes some of
the visitors that were easy to fix.
Differential Revision: https://phabricator.services.mozilla.com/D70788
This used to be needed for Gecko interop, but now all this is in the Rust side
so we no longer need it.
Depends on D63861
Differential Revision: https://phabricator.services.mozilla.com/D63863
The heuristic is that we show focus outlines for unknown or key focus, and not
for mouse / touch.
This is probably not the final heuristic we take, but this allows people to play
with it and file bugs.
Once this is mature enough we should remove :-moz-focusring in favor of
:focus-visible.
Differential Revision: https://phabricator.services.mozilla.com/D63861
We need to ensure the rules that override all properties for scrollbar
part elements only apply to those that are NAC (and so will be eligible
for NAC style sharing). We have some uses of non-NAC <scrollbar>
elements that should continue to inherit properties from their parents.
To avoid any changes in rule matching order that come with changing specificity,
we add a new :-moz-native-anonymous-no-specificity pseudo-class.
While we're here, we note :-moz-native-anonymous-no-specificity (and the
regular :-moz-native-anonymous pseudo-class) as not needing style
sharing cache revalidation, as we never share NAC styles.
Differential Revision: https://phabricator.services.mozilla.com/D56154
It's much nicer.
One nice thing about this is that the new code is subject to the existing
threadedness checking, which identified that several of these should be atomic
because they're accessed off the main thread.
Differential Revision: https://phabricator.services.mozilla.com/D40792
This doesn't change the way C++ code uses static prefs. But it does slightly
change how Rust code uses static prefs, specifically the name generated by
bindgen is slightly different.
The commit also improves some comments.
Differential Revision: https://phabricator.services.mozilla.com/D35764
I want to enable in Nightly to evaluate (in the medium term) shipping it without
the part forwarding, once the cascade order and importance issues are fixed, and
that we pass all the tests that don't involve forwarding.
That is, I want to monitor whether having ::part() causes compat issues or not.
Differential Revision: https://phabricator.services.mozilla.com/D32649
