/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this file,
 * You can obtain one at http://mozilla.org/MPL/2.0/. */

#include "ViewTimeline.h"

#include "mozilla/Keyframe.h"
#include "mozilla/ScrollContainerFrame.h"
#include "mozilla/ServoCSSParser.h"
#include "mozilla/ServoStyleSet.h"
#include "mozilla/dom/Animation.h"
#include "mozilla/dom/CSSKeywordValue.h"
#include "mozilla/dom/CSSUnitValue.h"
#include "mozilla/dom/Document.h"
#include "mozilla/dom/DocumentInlines.h"
#include "mozilla/dom/ElementInlines.h"
#include "mozilla/dom/TimelineName.h"
#include "mozilla/dom/ViewTimelineBinding.h"
#include "nsComputedDOMStyle.h"
#include "nsLayoutUtils.h"
#include "nsPresContext.h"

namespace mozilla::dom {

NS_IMPL_CYCLE_COLLECTION_INHERITED(ViewTimeline, ScrollTimeline, mSubject)
NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED_0(ViewTimeline, ScrollTimeline)

/* static */
already_AddRefed<ViewTimeline> ViewTimeline::MakeNamed(
    Document* aDocument, Element* aSubject,
    const PseudoStyleRequest& aPseudoRequest, StyleScrollAxis aAxis,
    const StyleViewTimelineInset& aInset) {
  MOZ_ASSERT(NS_IsMainThread());

  // 1. Create an anonymous scroller, as if `scroll(nearest)`.
  auto scroller = ScrollerInfo::Anonymous(
      StyleScroller::Nearest,
      NonOwningAnimationTarget{aSubject, aPseudoRequest});

  // 2. Create timeline.
  return MakeAndAddRef<ViewTimeline>(aDocument, scroller, aAxis, aSubject,
                                     aPseudoRequest.mType, aInset, false);
}

/* static */
already_AddRefed<ViewTimeline> ViewTimeline::MakeAnonymous(
    Document* aDocument, const NonOwningAnimationTarget& aTarget,
    StyleScrollAxis aAxis, const StyleViewTimelineInset& aInset) {
  // view() finds the nearest scroll container from the animation target.
  auto scroller = ScrollerInfo::Anonymous(StyleScroller::Nearest, aTarget);
  return MakeAndAddRef<ViewTimeline>(
      aDocument, scroller, aAxis, aTarget.mElement,
      aTarget.mPseudoRequest.mType, aInset, true);
}

JSObject* ViewTimeline::WrapObject(JSContext* aCx,
                                   JS::Handle<JSObject*> aGivenProto) {
  return ViewTimeline_Binding::Wrap(aCx, this, aGivenProto);
}

static Maybe<StyleLengthPercentageOrAuto> ComputeStartOrEndInset(
    const OwningUTF8StringOrCSSKeywordValueOrCSSNumericValue& aValue,
    ErrorResult& aRv) {
  // Note: This is a CSSKeywordish, so we don't expect to accept a length or
  // percentage for this string.
  // Note: We compare the string case-insensitively to make the behavior
  // consistent with ServoCSSParser::ParseViewTimelineInset().
  // https://github.com/w3c/csswg-drafts/issues/9584
  if (aValue.IsUTF8String()) {
    const nsCString& s = aValue.GetAsUTF8String();
    if (!s.LowerCaseEqualsLiteral("auto")) {
      aRv.ThrowTypeError("Invalid inset keyword");
      return Nothing();
    }
    return Some(StyleLengthPercentageOrAuto::Auto());
  } else if (aValue.IsCSSKeywordValue()) {
    nsAutoCString s;
    aValue.GetAsCSSKeywordValue()->GetValue(s);
    if (!s.LowerCaseEqualsLiteral("auto")) {
      aRv.ThrowTypeError("Invalid inset keyword");
      return Nothing();
    }
    return Some(StyleLengthPercentageOrAuto::Auto());
  }

  nsAutoCString s;
  const CSSNumericValue& numeric = aValue.GetAsCSSNumericValue();
  numeric.Stringify(s);
  StyleLengthPercentage result;
  if (!ServoCSSParser::ParseLengthPercentageForAbsoluteLengths(s, result)) {
    aRv.ThrowTypeError("Invalid inset value");
    return Nothing();
  }
  return Some(StyleLengthPercentageOrAuto::LengthPercentage(result));
}

/* static */
already_AddRefed<ViewTimeline> ViewTimeline::Constructor(
    const GlobalObject& aGlobal, const ViewTimelineOptions& aOptions,
    ErrorResult& aRv) {
  RefPtr<Document> doc =
      AnimationUtils::GetCurrentRealmDocument(aGlobal.Context());
  if (!doc) {
    aRv.Throw(NS_ERROR_FAILURE);
    return nullptr;
  }

  // The spec doesn't provide the default value for element, so we use null
  // subject to align the behavior with other browsers.
  RefPtr<Element> subject = aOptions.mSubject;

  StyleScrollAxis axis;
  switch (aOptions.mAxis) {
    case dom::ScrollAxis::Block:
      axis = StyleScrollAxis::Block;
      break;
    case dom::ScrollAxis::Inline:
      axis = StyleScrollAxis::Inline;
      break;
    case dom::ScrollAxis::X:
      axis = StyleScrollAxis::X;
      break;
    case dom::ScrollAxis::Y:
      axis = StyleScrollAxis::Y;
      break;
  }

  StyleViewTimelineInset inset;
  if (aOptions.mInset.IsUTF8String()) {
    // If a DOMString value is provided as an inset, parse it as a
    // <'view-timeline-inset'> value;
    if (!ServoCSSParser::ParseViewTimelineInset(
            aOptions.mInset.GetAsUTF8String(), inset)) {
      // We throw TypeError for the invalid inset, including DOMString, just
      // like the invalid sequence case per spec.
      aRv.ThrowTypeError("Invalid inset string");
      return nullptr;
    }
  } else {
    if (!StaticPrefs::layout_css_typed_om_enabled()) {
      // CSSKeywordValue and CSSNumericValue are disabled.
      aRv.Throw(NS_ERROR_DOM_NOT_SUPPORTED_ERR);
      return nullptr;
    }
    // if a sequence is provided, the first value represents the start inset and
    // the second value represents the end inset. If the sequence has only one
    // value, it is duplicated. If it has zero values or more than two values,
    // or if it contains a CSSKeywordValue whose value is not "auto", throw a
    // TypeError.
    const auto& sequence =
        aOptions.mInset
            .GetAsUTF8StringOrCSSKeywordValueOrCSSNumericValueSequence();
    if (sequence.Length() == 0 || sequence.Length() > 2) {
      aRv.ThrowTypeError("Invalid inset sequence");
      return nullptr;
    }
    auto start = ComputeStartOrEndInset(sequence[0], aRv);
    if (!start) {
      return nullptr;
    }
    auto end = sequence.Length() == 2 ? ComputeStartOrEndInset(sequence[1], aRv)
                                      : start;
    if (!end) {
      return nullptr;
    }
    inset.start = std::move(*start);
    inset.end = std::move(*end);
  }

  // Set the source of timeline to the subject’s nearest ancestor scroll
  // container element.
  // Note: if subject is null, we use null source as well.
  ScrollerInfo scroller = ScrollerInfo::Anonymous(
      subject ? ScrollerInfo::Type::Nearest : ScrollerInfo::Type::Provided,
      subject, PseudoStyleRequest::NotPseudo());

  RefPtr<ViewTimeline> result = MakeAndAddRef<ViewTimeline>(
      doc, scroller, axis, subject, PseudoStyleType::NotPseudo, inset, false);
  if (subject) {
    // The values of subject, source, and currentTime are all computed when any
    // of them is requested or updated, per spec.
    if (Document* doc = subject->GetComposedDoc()) {
      doc->FlushPendingNotifications(FlushType::Layout);
    }
    // Maybe our nearested scroller already exists, try to compute the current
    // time.
    result->UpdateCachedCurrentTime();
  }

  return result.forget();
}

already_AddRefed<CSSNumericValue> ViewTimeline::GetStartOffset(
    ErrorResult& aRv) const {
  auto data = ComputeTimelineData();
  if (!data) {
    return nullptr;
  }

  if (!StaticPrefs::layout_css_typed_om_enabled()) {
    aRv.Throw(NS_ERROR_DOM_NOT_SUPPORTED_ERR);
    return nullptr;
  }
  return MakeCSSUnitValue(
      GetParentObject(), StyleNumericType::Length(),
      nsPresContext::AppUnitsToDoubleCSSPixels(data->mStart), "px"_ns);
}

already_AddRefed<CSSNumericValue> ViewTimeline::GetEndOffset(
    ErrorResult& aRv) const {
  auto data = ComputeTimelineData();
  if (!data) {
    return nullptr;
  }

  if (!StaticPrefs::layout_css_typed_om_enabled()) {
    aRv.Throw(NS_ERROR_DOM_NOT_SUPPORTED_ERR);
    return nullptr;
  }
  return MakeCSSUnitValue(GetParentObject(), StyleNumericType::Length(),
                          nsPresContext::AppUnitsToDoubleCSSPixels(data->mEnd),
                          "px"_ns);
}

void ViewTimeline::ReplacePropertiesWith(
    Element* aSubjectElement, const PseudoStyleRequest& aPseudoRequest,
    const dom::ScopedTimelineName& aName, StyleScrollAxis aAxis,
    const StyleViewTimelineInset& aInset) {
  mSubject = aSubjectElement;
  mSubjectPseudoType = aPseudoRequest.mType;
  mAxis = aAxis;
  // FIXME: Bug 1817073. We assume it is a non-animatable value for now.
  mInset = aInset;

  for (auto* anim = mAnimationOrder.getFirst(); anim;
       anim = static_cast<LinkedListElement<Animation>*>(anim)->getNext()) {
    MOZ_ASSERT(anim->GetTimeline() == this);
    MOZ_ASSERT(anim->GetTimelineName() == aName);
    // Set this so we just PostUpdate() for this animation.
    // FIXME(dshin, bug 1737927): Mutation observer may need to be notified.
    anim->SetTimeline(this, aName, Animation::FromJS::No);
  }
}

static std::pair<nscoord, nscoord> ComputeInsets(
    const ScrollContainerFrame* aScrollContainerFrame,
    const layers::ScrollDirection aOrientation, const StyleScrollAxis aAxis,
    const StyleViewTimelineInset& aInset) {
  // If view-timeline-inset is auto, it indicates to use the value of
  // scroll-padding. We use logical dimension to map that start/end offset to
  // the corresponding scroll-padding-{inline|block}-{start|end} values.
  const WritingMode wm =
      aScrollContainerFrame->GetScrolledFrame()->GetWritingMode();
  const auto& scrollPadding =
      LogicalMargin(wm, aScrollContainerFrame->GetScrollPadding());
  const bool isBlockAxis = aAxis == StyleScrollAxis::Block ||
                           (aAxis == StyleScrollAxis::X && wm.IsVertical()) ||
                           (aAxis == StyleScrollAxis::Y && !wm.IsVertical());

  // The percentages of view-timelne-inset is relative to the corresponding
  // dimension of the relevant scrollport.
  // https://drafts.csswg.org/scroll-animations-1/#view-timeline-inset
  const nsRect scrollPort = aScrollContainerFrame->GetScrollPortRect();
  const nscoord percentageBasis =
      aOrientation == layers::ScrollDirection::eHorizontal ? scrollPort.width
                                                           : scrollPort.height;

  nscoord startInset =
      aInset.start.IsAuto()
          ? (isBlockAxis ? scrollPadding.BStart(wm) : scrollPadding.IStart(wm))
          : aInset.start.AsLengthPercentage().Resolve(percentageBasis);
  nscoord endInset =
      aInset.end.IsAuto()
          ? (isBlockAxis ? scrollPadding.BEnd(wm) : scrollPadding.IEnd(wm))
          : aInset.end.AsLengthPercentage().Resolve(percentageBasis);
  return {startInset, endInset};
}

bool ViewTimeline::UpdateCachedCurrentTime() {
  const auto prevCachedCurrentTime = std::move(mCachedCurrentTime);

  mCachedCurrentTime.reset();

  mCachedStateSnapshot = Some(ComputeSnapshot());
  // The timeline is inactive if it has no principal box or its source is not a
  // scroll container.
  if (!mCachedStateSnapshot->IsActive()) {
    return prevCachedCurrentTime.isSome();
  }

  const ScrollContainerFrame* scrollContainerFrame =
      mCachedStateSnapshot->GetScrollContainerFrame();
  MOZ_ASSERT(scrollContainerFrame);

  // Don't try to update against a frame that hasn't been laid out yet.
  if (scrollContainerFrame->HasAnyStateBits(NS_FRAME_FIRST_REFLOW)) {
    return prevCachedCurrentTime.isSome();
  }

  // Note: We may fail to get the pseudo element (or its primary frame) if it is
  // not generated yet or just get destroyed, while we are sampling this view
  // timeline.
  // FIXME: Bug 1954230. It's probably a case we need to discard this timeline.
  // For now, this is just a hot fix.
  MOZ_ASSERT(mSubject, "We should have a subject to create this view timeline");
  const Element* subjectElement =
      mSubject->GetPseudoElement(PseudoStyleRequest(mSubjectPseudoType));
  const nsIFrame* subject =
      subjectElement ? subjectElement->GetPrimaryFrame() : nullptr;
  if (!subject) {
    // No principal box of the subject, so we cannot compute the offset. This
    // may happen when we clear all animation collections during unbinding from
    // the tree.
    return prevCachedCurrentTime.isSome();
  }

  // The current scroll position and scroll range.
  const nsPoint& scrollPosition = scrollContainerFrame->GetScrollPosition();
  const nsRect& scrollRange = scrollContainerFrame->GetScrollRange();

  // In order to get the distance between the subject and the scrollport
  // properly, we use the position based on the domain of the scrolled frame,
  // instead of the scroll container frame.
  const nsIFrame* scrolledFrame = scrollContainerFrame->GetScrolledFrame();
  MOZ_ASSERT(scrolledFrame);
  const nsRect subjectRect(subject->GetOffsetTo(scrolledFrame),
                           subject->GetSize());

  // Use scrollport size (i.e. padding box size - scrollbar size), which is used
  // for calculating the view progress visibility range.
  // https://drafts.csswg.org/scroll-animations/#view-progress-visibility-range
  const nsRect scrollPort = scrollContainerFrame->GetScrollPortRect();

  // |sideInsets.mEnd| is used to adjust the start offset, and
  // |sideInsets.mStart| is used to adjust the end offset. This is because
  // |sideInsets.mStart| refers to logical start side [1] of the source box
  // (i.e. the box of the scrollport), where as |startOffset| refers to the
  // start of the timeline, and similarly for end side/offset. [1]
  // https://drafts.csswg.org/css-writing-modes-4/#css-start
  const auto orientation = mCachedStateSnapshot->Axis();
  const auto sideInsets =
      ComputeInsets(scrollContainerFrame, orientation, mAxis, mInset);

  // Adjuct the positions and sizes based on the physical axis.
  const WritingMode wm = scrolledFrame->GetWritingMode();
  switch (orientation) {
    case layers::ScrollDirection::eVertical: {
      // Mirror of the R-L case below for bottom-to-top scrolling (vertical
      // writing-mode + direction:rtl), where the inline axis is vertical and
      // reversed, so scrollPosition.y is zero or negative.
      const bool isBottomToTop = wm.IsVertical() && wm.IsInlineReversed();
      mCachedCurrentTime.emplace(CurrentTimeData{
          ScrollTimeline::CurrentTimeData{scrollPosition.y, scrollRange.height},
          scrollPort.height,
          isBottomToTop ? scrolledFrame->GetSize().height - subjectRect.YMost()
                        : subjectRect.y,
          subjectRect.height, sideInsets.first, sideInsets.second});
      break;
    }
    case layers::ScrollDirection::eHorizontal:
      mCachedCurrentTime.emplace(CurrentTimeData{
          ScrollTimeline::CurrentTimeData{scrollPosition.x, scrollRange.width},
          scrollPort.width,
          // |mSubjectPosition| should be the position of the start border edge
          // of the subject, so for R-L case, we have to use XMost() as the
          // start border edge of the subject, and compute its position by using
          // the x-most side of the scrolled frame as the origin on the
          // horizontal axis.
          wm.IsPhysicalRTL()
              ? scrolledFrame->GetSize().width - subjectRect.XMost()
              : subjectRect.x,
          subjectRect.width, sideInsets.first, sideInsets.second});
      break;
  }

  if (!prevCachedCurrentTime ||
      prevCachedCurrentTime->IsChanged(*mCachedCurrentTime)) {
    TimelineDataDidChange();
  }
  return mCachedCurrentTime != prevCachedCurrentTime;
}

// FIXME: Bug 2018678. Need to be adjusted for sticky positioning element.
// https://drafts.csswg.org/scroll-animations-1/#view-timelines-ranges
std::pair<nscoord, nscoord> ViewTimeline::IntervalForTimelineRangeName(
    const StyleTimelineRangeName aName,
    const ScrollTimeline::ComputedTimelineData& aData) const {
  MOZ_ASSERT(mCachedCurrentTime, "We should have a cached current time");

  // The following variable names are based on the vertical scrolling direction
  // and the subject becomes visible from the bottom of the scroll port.

  // The scroll offset when we align the start border edge of the subject with
  // the end edge of the scroll port.
  const nscoord alignedSubjectStartViewEnd = aData.mStart;
  // The scroll offset when we align the end border edge of the subject with
  // the start edge of the scroll port.
  const nscoord alignedSubjectEndViewStart = aData.mEnd;
  // The scroll offset when we align the start border edge of the subject with
  // the start edge of the scroll port.
  const nscoord alignedSubjectStartViewStart =
      alignedSubjectEndViewStart - mCachedCurrentTime->mSubjectSize;
  // The scroll offset when we align the end border edge of the subject with the
  // end edge of the scroll port.
  const nscoord alignedSubjectEndViewEnd =
      alignedSubjectStartViewEnd + mCachedCurrentTime->mSubjectSize;

  // Precompute the range of `contain` to avoid the code duplication. See below
  // for more details.
  const nscoord containStart =
      std::min(alignedSubjectStartViewStart, alignedSubjectEndViewEnd);
  const nscoord containEnd =
      std::max(alignedSubjectStartViewStart, alignedSubjectEndViewEnd);

  // FIXME: Bug 2030453. Check the case for RTL for horizontal axis. Perhaps we
  // have to swap these two values.
  switch (aName) {
    case StyleTimelineRangeName::None:
    case StyleTimelineRangeName::Normal:
      // The default behavior is equalivant to `cover` for view timeline.
    case StyleTimelineRangeName::Cover:
      // Represents the full range of the view progress timeline:
      // * 0% progress represents the latest position at which the start border
      //   edge of the element’s principal box coincides with the end edge of
      //   its view progress visibility range.
      // * 100% progress represents the earliest position at which the end
      //   border edge of the element’s principal box coincides with the start
      //   edge of its view progress visibility range.
      return {alignedSubjectStartViewEnd, alignedSubjectEndViewStart};

    case StyleTimelineRangeName::Contain:
      // Represents the range during which the principal box is either fully
      // contained by, or fully covers, its view progress visibility range
      // within the scrollport.
      // 0% progress represents the earliest position at which either:
      //   1. the start border edge of the element’s principal box coincides
      //      with the start edge of its view progress visibility range.
      //   2. the end border edge of the element’s principal box coincides with
      //      the end edge of its view progress visibility range.
      // 100% progress represents the latest position at which either:
      //   1. the start border edge of the element’s principal box coincides
      //      with the start edge of its view progress visibility range.
      //   2. the end border edge of the element’s principal box coincides with
      //      the end edge of its view progress visibility range.
      //
      // Note that we swap the values if the subject size is larger than the
      // scrollport size. That's why there are 2 options for 0% and 2 options
      // for 100% in the spec.
      //
      // For more visual explanation, see:
      // https://github.com/w3c/csswg-drafts/issues/7973#issuecomment-1427150014
      return {containStart, containEnd};

    case StyleTimelineRangeName::Entry:
      // Represents the range during which the principal box is entering the
      // view progress visibility range.
      // * 0% is equivalent to 0% of the cover range.
      // * 100% is equivalent to 0% of the contain range.
      return {alignedSubjectStartViewEnd, containStart};

    case StyleTimelineRangeName::Exit:
      // Represents the range during which the principal box is exiting the view
      // progress visibility range.
      // * 0% is equivalent to 100% of the contain range.
      // * 100% is equivalent to 100% of the cover range.
      return {containEnd, alignedSubjectEndViewStart};

    case StyleTimelineRangeName::EntryCrossing:
      // Represents the range during which the principal box crosses the end
      // border edge.
      // * 0% is equivalent to 0% of the cover range.
      //
      // Note that the duration of the entry-crossing range is equal to the
      // subject size, so this is equivalent to
      // `{alignedSubjectStartViewEnd,
      //   alignedSubjectStartViewEnd + mCachedCurrentTime->mSubjectSize}`.
      return {alignedSubjectStartViewEnd, alignedSubjectEndViewEnd};

    case StyleTimelineRangeName::ExitCrossing:
      // Represents the range during which the principal box crosses the start
      // border edge.
      // * 100% is equivalent to 100% of the cover range.
      //
      // Note that the duration of the exit-crossing range is equal to the
      // subject size, so this is equivalent to
      // `{alignedSubjectEndViewStart - mCachedCurrentTime->mSubjectSize,
      //   alignedSubjectEndViewStart}`.
      return {alignedSubjectStartViewStart, alignedSubjectEndViewStart};

    case StyleTimelineRangeName::Scroll:
      // Represents the full range of the scroll container on which the view
      // progress timeline is defined.
      //
      // So this is equivalent to scroll timeline's full range.
      return {0, mCachedCurrentTime->mScrollData.mMaxScrollOffset};
  }

  MOZ_ASSERT_UNREACHABLE("All cases should be handled.");
  // Use cover as the default value. However, we shouldn't be here.
  return {alignedSubjectStartViewEnd, alignedSubjectEndViewStart};
}

// Calculate the offset (as a percentage) for a pair of range name and offset,
// based on the full timeline range (i.e. `cover` for view-timeline).
template <typename F>
double ViewTimeline::ComputeOffsetToTimelineRange(
    const StyleTimelineRangeName& aName,
    const ScrollTimeline::ComputedTimelineData& aData,
    F&& aFuncToResolveValue) const {
  const auto [nameStart, nameEnd] = IntervalForTimelineRangeName(aName, aData);
  const auto timelineRange = aData.mEnd - aData.mStart;
  const auto nameRange = nameEnd - nameStart;
  const auto positionInNameRange = nameStart + aFuncToResolveValue(nameRange);
  const auto positionInTimeline = positionInNameRange - aData.mStart;
  return static_cast<double>(positionInTimeline) /
         static_cast<double>(timelineRange);
}

Maybe<double> ViewTimeline::MapKeyframeOffsetToOffset(
    const StyleTimelineRangeName aName, const double aPercentage) const {
  const auto& data = ComputeTimelineData();
  if (!data) {
    return Nothing();
  }

  return Some(ComputeOffsetToTimelineRange(
      aName, *data,
      [&](const nscoord aBasis) { return aPercentage * aBasis; }));
}

std::pair<double, double> ViewTimeline::IntervalForAttachmentRange(
    const AnimationRange& aStyleRange) const {
  const auto& data = ComputeTimelineData();
  if (!data) {
    // Return the default, [0%, 100%].
    return {0, 1.0};
  }

  // Returns the percentage (in double) for this StyleAnimationValue based on
  // the full timeline range (i.e. `cover` for view-timeline).
  auto computeNamedRangeEdgeAsPercentage =
      [&](const StyleGenericAnimationRangeValue<StyleLengthPercentage>&
              aValue) {
        return ComputeOffsetToTimelineRange(
            aValue.name, *data,
            [&](const nscoord aBasis) { return aValue.lp.Resolve(aBasis); });
      };
  return {computeNamedRangeEdgeAsPercentage(aStyleRange.mStart),
          computeNamedRangeEdgeAsPercentage(aStyleRange.mEnd)};
}

bool ViewTimeline::IsReusableAnonymousTimeline(
    const StyleGenericViewFunction<StyleLengthPercentage>& aView) const {
  if (!mIsAnonymous) {
    return false;
  }
  return mAxis == aView.axis && mInset == aView.inset;
}

Maybe<ScrollTimeline::ComputedTimelineData> ViewTimeline::ComputeTimelineData()
    const {
  if (!mCachedCurrentTime) {
    return Nothing();
  }

  const CurrentTimeData& data = mCachedCurrentTime.ref();

  // We use "cover" timeline range as the default full range for view
  // timeline.
  // https://drafts.csswg.org/scroll-animations-1/#view-timeline-progress

  // Note: `mSubjectPosition - mScrollPortSize` means the distance between the
  // start border edge of the subject and the end edge of the scrollport.
  const nscoord startOffset =
      data.mSubjectPosition - data.mScrollPortSize + data.mInsetEnd;
  // Note: `mSubjectPosition + mSubjectSize` means the position of the end
  // border edge of the subject. When it touches the start edge of the
  // scrollport, it is 100%.
  const nscoord endOffset =
      data.mSubjectPosition + data.mSubjectSize - data.mInsetStart;

  return Some(ComputedTimelineData{
      data.mScrollData.mPosition,
      startOffset,
      endOffset,
  });
}

}  // namespace mozilla::dom