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
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
* 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>
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
Instead of applying animations and transitions to styled elements,
include them in the cascade. This allows them to interact properly with
things like font-size and !important rules.
Do the same we do for classes for now. We could be more precise and achieve a
bit more sharing with some more effort (left a comment there), but it seems
unlikely to matter in practice (and if we did that, we'd probably want to do the
same for classes).
Differential Revision: https://phabricator.services.mozilla.com/D58453
Remove unused code (4/4)
<!-- Please describe your changes on the following line: -->
Fourth and final PR in a series of PRs to remove unused/dead code from servo, powered by an (upcoming) tool of mine. Please take a look and tell me if you want to keep something.
* First PR: #23477
* Second PR: #23498
* Third PR: #23499
Shortstat of the combined PR series:
```
47 files changed, 7 insertions(+), 805 deletions(-)
```
---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because they only remove dead code
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---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/23532)
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We already avoid putting styles in the shared style cache for an element that
has animations[1] but if we are doing the initial style of two siblings where
the _second_ has animations applied, there is nothing to stop us from trying to
share with the first (un-animated) element. This patch adds a check to prevent
sharing in that case since sharing style between animated elements is unsound
for the reasons described in [1].
A number of tests including:
testing/web-platform/tests/web-animations/animation-model/animation-types/visibility.html
will fail without this fix once we remove the style flush from
KeyframeEffect::SetKeyframes later in this patch series.
[1] https://searchfox.org/mozilla-central/rev/6e3cc153566f5f288ae768a2172385b8436d61dd/servo/components/style/sharing/mod.rs#597
Differential Revision: https://phabricator.services.mozilla.com/D18913
We were spuriously reframing the <shadow> because it initially shared style with
the <br>, which ended up being display: none, while the <shadow> should've been
display: contents from the beginning.
lookup_by_rules seems pretty prone to obscure bugs, and also it's pretty
complex... Probably we should try to get rid of it, I'm unconvinced that it's
worth it.
Even with that, in a normal restyle the <details> wouldn't have ended up with a
style. It of course never had it before the reframe because the <shadow> was
display: none, but that doesn't mean it shouldn't have gotten one, since we
detected we needed to go through kids in:
https://searchfox.org/mozilla-central/rev/6eea08365e7386a2b81c044e7cc8a3daa51d8754/servo/components/style/matching.rs#500
That code did happen, but since it's an animation-only restyle, we don't look at
unstyled stuff.
That looks somewhat fishy, but I guess for now it's fine as long as display
isn't animatable.
Bug: 1469076
Reviewed-by: heycam
MozReview-Commit-ID: B6NMSTNOKgK
Currently, NAC always inherits from the closest non-NAC ancestor element,
regardless of whether it is for an element-backed pseudo or not.
This patch changes the inheritance so that for element-backed pseudos, we
inherit from the closest native anonymous root's parent, and for other NAC we
inherit from the parent.
This prevents the following two issues and allows us to remove the
NODE_IS_NATIVE_ANONYMOUS flag:
* Avoiding inheriting from the non-NAC ancestor in XBL bindings bound to NAC.
- This is no longer a problem since we apply the rule only if we're a
pseudo-element, and all pseudo-elements are in native anonymous subtrees.
- This also allows to remove the hack that propagates the
NODE_IS_NATIVE_ANONYMOUS flag from the ::cue pseudo-element from
BindToTree.
* Inheriting from the wrong thing if we're a nested NAC subtree.
- We no longer look past our NAC subtree, with the exception of
::-moz-number-text's pseudo-elements, for which we do want to propagate
::placeholder to.
A few rules from forms.css have been modified because they're useless or needed
to propagate stuff to the anonymous form control in input[type="number"] which
previously inherited from the input itself.
Bug: 1460382
Reviewed-by: heycam
MozReview-Commit-ID: IDKYt3EJtSH
Kinda tricky because :host only matches rules on the shadow root where the rules
come from. So we need to be careful during invalidation and style sharing.
I didn't use the non_ts_pseudo_class_list bits because as soon as we implement
the :host(..) bits we're going to need to special-case it anyway.
The general schema is the following:
* Rightmost featureless :host selectors are handled inserting them in the
host_rules hashmap. Note that we only insert featureless stuff there. We
could insert all of them and just filter during matching, but that's slightly
annoying.
* The other selectors, like non-featureless :host or what not, are added to the
normal cascade data. This is harmless, since the shadow host rules are never
matched against the host, so we know they'll just never match, and avoids
adding more special-cases.
* Featureless :host selectors to the left of a combinator are handled during
matching, in the special-case of next_element_for_combinator in selectors.
This prevents this from being more invasive, and keeps the usual fast path
slim, but it's a bit hard to match the spec and the implementation.
We could keep a copy of the SelectorIter instead in the matching context to
make the handling of featureless-ness explicit in match_non_ts_pseudo_class,
but we'd still need the special-case anyway, so I'm not fond of it.
* We take advantage of one thing that makes this sound. As you may have
noticed, if you had `root` element which is a ShadowRoot, and you matched
something like `div:host` against it, using a MatchingContext with
current_host == root, we'd incorrectly report a match. But this is impossible
due to the following constraints:
* Shadow root rules aren't matched against the host during styling (except
these featureless selectors).
* DOM APIs' current_host needs to be the _containing_ host, not the element
itself if you're a Shadow host.
Bug: 992245
Reviewed-by: xidorn
MozReview-Commit-ID: KayYNfTXb5h
If two elements are assigned to different slots, even within the same shadow
root, they could match different rules, due to the slot being assigned to yet
another slot in another shadow root.
MozReview-Commit-ID: CfZJCVxIQzM
