ScrollSnap.cpp
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* 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 "ScrollSnap.h"
#include "FrameMetrics.h"
#include "mozilla/ServoStyleConsts.h"
#include "nsIFrame.h"
#include "nsIScrollableFrame.h"
#include "nsLayoutUtils.h"
#include "nsPresContext.h"
#include "nsTArray.h"
#include "mozilla/StaticPrefs_layout.h"
namespace mozilla {
using layers::ScrollSnapInfo;
/**
* Keeps track of the current best edge to snap to. The criteria for
* adding an edge depends on the scrolling unit.
*/
class CalcSnapPoints final {
public:
CalcSnapPoints(ScrollUnit aUnit, ScrollSnapFlags aSnapFlags,
const nsPoint& aDestination, const nsPoint& aStartPos);
struct SnapPosition {
SnapPosition() = default;
SnapPosition(nscoord aPosition, StyleScrollSnapStop aScrollSnapStop,
ScrollSnapTargetId aTargetId)
: mPosition(aPosition),
mScrollSnapStop(aScrollSnapStop),
mTargetId(aTargetId) {}
nscoord mPosition;
StyleScrollSnapStop mScrollSnapStop;
ScrollSnapTargetId mTargetId;
};
void AddHorizontalEdge(const SnapPosition& aEdge);
void AddVerticalEdge(const SnapPosition& aEdge);
struct CandidateTracker {
explicit CandidateTracker(nscoord aDestination)
: mBestEdge(SnapPosition{aDestination, StyleScrollSnapStop::Normal,
ScrollSnapTargetId::None}) {
// We use NSCoordSaturatingSubtract to calculate the distance between a
// given position and this second best edge position so that it can be an
// uninitialized value as the maximum possible value, because the first
// distance calculation would always be nscoord_MAX.
mSecondBestEdge = SnapPosition{nscoord_MAX, StyleScrollSnapStop::Normal,
ScrollSnapTargetId::None};
mEdgeFound = false;
}
// keeps track of the position of the current best edge on this axis.
SnapPosition mBestEdge;
// keeps track of the position of the current second best edge on the
// opposite side of the best edge on this axis.
SnapPosition mSecondBestEdge;
bool mEdgeFound; // true if mBestEdge is storing a valid edge.
// Assuming in most cases there's no multiple coincide snap points.
AutoTArray<ScrollSnapTargetId, 1> mTargetIds;
};
void AddEdge(const SnapPosition& aEdge, nscoord aDestination,
nscoord aStartPos, nscoord aScrollingDirection,
CandidateTracker* aCandidateTracker);
SnapTarget GetBestEdge() const;
nscoord XDistanceBetweenBestAndSecondEdge() const {
return std::abs(NSCoordSaturatingSubtract(
mTrackerOnX.mSecondBestEdge.mPosition, mTrackerOnX.mBestEdge.mPosition,
nscoord_MAX));
}
nscoord YDistanceBetweenBestAndSecondEdge() const {
return std::abs(NSCoordSaturatingSubtract(
mTrackerOnY.mSecondBestEdge.mPosition, mTrackerOnY.mBestEdge.mPosition,
nscoord_MAX));
}
const nsPoint& Destination() const { return mDestination; }
protected:
ScrollUnit mUnit;
ScrollSnapFlags mSnapFlags;
nsPoint mDestination; // gives the position after scrolling but before
// snapping
nsPoint mStartPos; // gives the position before scrolling
nsIntPoint mScrollingDirection; // always -1, 0, or 1
CandidateTracker mTrackerOnX;
CandidateTracker mTrackerOnY;
};
CalcSnapPoints::CalcSnapPoints(ScrollUnit aUnit, ScrollSnapFlags aSnapFlags,
const nsPoint& aDestination,
const nsPoint& aStartPos)
: mUnit(aUnit),
mSnapFlags(aSnapFlags),
mDestination(aDestination),
mStartPos(aStartPos),
mTrackerOnX(aDestination.x),
mTrackerOnY(aDestination.y) {
MOZ_ASSERT(aSnapFlags != ScrollSnapFlags::Disabled);
nsPoint direction = aDestination - aStartPos;
mScrollingDirection = nsIntPoint(0, 0);
if (direction.x < 0) {
mScrollingDirection.x = -1;
}
if (direction.x > 0) {
mScrollingDirection.x = 1;
}
if (direction.y < 0) {
mScrollingDirection.y = -1;
}
if (direction.y > 0) {
mScrollingDirection.y = 1;
}
}
SnapTarget CalcSnapPoints::GetBestEdge() const {
return SnapTarget{
nsPoint(
mTrackerOnX.mEdgeFound ? mTrackerOnX.mBestEdge.mPosition
// In the case of IntendedEndPosition (i.e. the destination point is
// explicitely specied, e.g. scrollTo) use the destination point if we
// didn't find any candidates.
: !(mSnapFlags & ScrollSnapFlags::IntendedDirection) ? mDestination.x
: mStartPos.x,
mTrackerOnY.mEdgeFound ? mTrackerOnY.mBestEdge.mPosition
// Same as above X axis case, use the destination point if we didn't
// find any candidates.
: !(mSnapFlags & ScrollSnapFlags::IntendedDirection) ? mDestination.y
: mStartPos.y),
ScrollSnapTargetIds{mTrackerOnX.mTargetIds, mTrackerOnY.mTargetIds}};
}
void CalcSnapPoints::AddHorizontalEdge(const SnapPosition& aEdge) {
AddEdge(aEdge, mDestination.y, mStartPos.y, mScrollingDirection.y,
&mTrackerOnY);
}
void CalcSnapPoints::AddVerticalEdge(const SnapPosition& aEdge) {
AddEdge(aEdge, mDestination.x, mStartPos.x, mScrollingDirection.x,
&mTrackerOnX);
}
void CalcSnapPoints::AddEdge(const SnapPosition& aEdge, nscoord aDestination,
nscoord aStartPos, nscoord aScrollingDirection,
CandidateTracker* aCandidateTracker) {
if (mSnapFlags & ScrollSnapFlags::IntendedDirection) {
// In the case of intended direction, we only want to snap to points ahead
// of the direction we are scrolling.
if (aScrollingDirection == 0 ||
(aEdge.mPosition - aStartPos) * aScrollingDirection <= 0) {
// The scroll direction is neutral - will not hit a snap point, or the
// edge is not in the direction we are scrolling, skip it.
return;
}
}
if (!aCandidateTracker->mEdgeFound) {
aCandidateTracker->mBestEdge = aEdge;
aCandidateTracker->mTargetIds =
AutoTArray<ScrollSnapTargetId, 1>{aEdge.mTargetId};
aCandidateTracker->mEdgeFound = true;
return;
}
auto isPreferredStopAlways = [&](const SnapPosition& aSnapPosition) -> bool {
MOZ_ASSERT(mSnapFlags & ScrollSnapFlags::IntendedDirection);
// In the case of intended direction scroll operations, `scroll-snap-stop:
// always` snap points in between the start point and the scroll destination
// are preferable preferable. In other words any `scroll-snap-stop: always`
// snap points can be handled as if it's `scroll-snap-stop: normal`.
return aSnapPosition.mScrollSnapStop == StyleScrollSnapStop::Always &&
std::abs(aSnapPosition.mPosition - aStartPos) <
std::abs(aDestination - aStartPos);
};
const bool isOnOppositeSide =
((aEdge.mPosition - aDestination) > 0) !=
((aCandidateTracker->mBestEdge.mPosition - aDestination) > 0);
const nscoord distanceFromStart = aEdge.mPosition - aStartPos;
// A utility function to update the best and the second best edges in the
// given conditions.
// |aIsCloserThanBest| True if the current candidate is closer than the best
// edge.
// |aIsCloserThanSecond| True if the current candidate is closer than
// the second best edge.
const nscoord distanceFromDestination = aEdge.mPosition - aDestination;
auto updateBestEdges = [&](bool aIsCloserThanBest, bool aIsCloserThanSecond) {
if (aIsCloserThanBest) {
if (mSnapFlags & ScrollSnapFlags::IntendedDirection &&
isPreferredStopAlways(aEdge)) {
// In the case of intended direction scroll operations and the new best
// candidate is `scroll-snap-stop: always` and if it's closer to the
// start position than the destination, thus we won't use the second
// best edge since even if the snap port of the best edge covers entire
// snapport, the `scroll-snap-stop: always` snap point is preferred than
// any points.
// NOTE: We've already ignored snap points behind start points so that
// we can use std::abs here in the comparison.
//
// For example, if there's a `scroll-snap-stop: always` in between the
// start point and destination, no `snap-overflow` mechanism should
// happen, if there's `scroll-snap-stop: always` further than the
// destination, `snap-overflow` might happen something like below
// diagram.
// start always dest other always
// |------------|---------|------|
aCandidateTracker->mSecondBestEdge = aEdge;
} else if (isOnOppositeSide) {
// Replace the second best edge with the current best edge only if the
// new best edge (aEdge) is on the opposite side of the current best
// edge.
aCandidateTracker->mSecondBestEdge = aCandidateTracker->mBestEdge;
}
aCandidateTracker->mBestEdge = aEdge;
aCandidateTracker->mTargetIds =
AutoTArray<ScrollSnapTargetId, 1>{aEdge.mTargetId};
} else {
if (aEdge.mPosition == aCandidateTracker->mBestEdge.mPosition) {
aCandidateTracker->mTargetIds.AppendElement(aEdge.mTargetId);
}
if (aIsCloserThanSecond && isOnOppositeSide) {
aCandidateTracker->mSecondBestEdge = aEdge;
}
}
};
bool isCandidateOfBest = false;
bool isCandidateOfSecondBest = false;
switch (mUnit) {
case ScrollUnit::DEVICE_PIXELS:
case ScrollUnit::LINES:
case ScrollUnit::WHOLE: {
isCandidateOfBest =
std::abs(distanceFromDestination) <
std::abs(aCandidateTracker->mBestEdge.mPosition - aDestination);
isCandidateOfSecondBest =
std::abs(distanceFromDestination) <
std::abs(NSCoordSaturatingSubtract(
aCandidateTracker->mSecondBestEdge.mPosition, aDestination,
nscoord_MAX));
break;
}
case ScrollUnit::PAGES: {
// distance to the edge from the scrolling destination in the direction of
// scrolling
nscoord overshoot = distanceFromDestination * aScrollingDirection;
// distance to the current best edge from the scrolling destination in the
// direction of scrolling
nscoord curOvershoot =
(aCandidateTracker->mBestEdge.mPosition - aDestination) *
aScrollingDirection;
nscoord secondOvershoot =
NSCoordSaturatingSubtract(
aCandidateTracker->mSecondBestEdge.mPosition, aDestination,
nscoord_MAX) *
aScrollingDirection;
// edges between the current position and the scrolling destination are
// favoured to preserve context
if (overshoot < 0) {
isCandidateOfBest = overshoot > curOvershoot || curOvershoot >= 0;
isCandidateOfSecondBest =
overshoot > secondOvershoot || secondOvershoot >= 0;
}
// if there are no edges between the current position and the scrolling
// destination the closest edge beyond the destination is used
if (overshoot > 0) {
isCandidateOfBest = overshoot < curOvershoot;
isCandidateOfSecondBest = overshoot < secondOvershoot;
}
}
}
if (mSnapFlags & ScrollSnapFlags::IntendedDirection) {
if (isPreferredStopAlways(aEdge)) {
// If the given position is `scroll-snap-stop: always` and if the position
// is in between the start and the destination positions, update the best
// position based on the distance from the __start__ point.
isCandidateOfBest =
std::abs(distanceFromStart) <
std::abs(aCandidateTracker->mBestEdge.mPosition - aStartPos);
} else if (isPreferredStopAlways(aCandidateTracker->mBestEdge)) {
// If we've found a preferable `scroll-snap-stop:always` position as the
// best, do not update it unless the given position is also
// `scroll-snap-stop: always`.
isCandidateOfBest = false;
}
}
updateBestEdges(isCandidateOfBest, isCandidateOfSecondBest);
}
static void ProcessSnapPositions(CalcSnapPoints& aCalcSnapPoints,
const ScrollSnapInfo& aSnapInfo) {
for (const auto& target : aSnapInfo.mSnapTargets) {
if (!target.IsValidFor(aCalcSnapPoints.Destination(),
aSnapInfo.mSnapportSize)) {
continue;
}
if (target.mSnapPositionX &&
aSnapInfo.mScrollSnapStrictnessX != StyleScrollSnapStrictness::None) {
aCalcSnapPoints.AddVerticalEdge(
{*target.mSnapPositionX, target.mScrollSnapStop, target.mTargetId});
}
if (target.mSnapPositionY &&
aSnapInfo.mScrollSnapStrictnessY != StyleScrollSnapStrictness::None) {
aCalcSnapPoints.AddHorizontalEdge(
{*target.mSnapPositionY, target.mScrollSnapStop, target.mTargetId});
}
}
}
Maybe<SnapTarget> ScrollSnapUtils::GetSnapPointForDestination(
const ScrollSnapInfo& aSnapInfo, ScrollUnit aUnit,
ScrollSnapFlags aSnapFlags, const nsRect& aScrollRange,
const nsPoint& aStartPos, const nsPoint& aDestination) {
if (aSnapInfo.mScrollSnapStrictnessY == StyleScrollSnapStrictness::None &&
aSnapInfo.mScrollSnapStrictnessX == StyleScrollSnapStrictness::None) {
return Nothing();
}
if (!aSnapInfo.HasSnapPositions()) {
return Nothing();
}
CalcSnapPoints calcSnapPoints(aUnit, aSnapFlags, aDestination, aStartPos);
ProcessSnapPositions(calcSnapPoints, aSnapInfo);
// If the distance between the first and the second candidate snap points
// is larger than the snapport size and the snapport is covered by larger
// elements, any points inside the covering area should be valid snap
// points.
// https://drafts.csswg.org/css-scroll-snap-1/#snap-overflow
// NOTE: |aDestination| sometimes points outside of the scroll range, e.g.
// by the APZC fling, so for the overflow checks we need to clamp it.
nsPoint clampedDestination = aScrollRange.ClampPoint(aDestination);
for (auto range : aSnapInfo.mXRangeWiderThanSnapport) {
if (range.IsValid(clampedDestination.x, aSnapInfo.mSnapportSize.width) &&
calcSnapPoints.XDistanceBetweenBestAndSecondEdge() >
aSnapInfo.mSnapportSize.width) {
calcSnapPoints.AddVerticalEdge(CalcSnapPoints::SnapPosition{
clampedDestination.x, StyleScrollSnapStop::Normal, range.mTargetId});
break;
}
}
for (auto range : aSnapInfo.mYRangeWiderThanSnapport) {
if (range.IsValid(clampedDestination.y, aSnapInfo.mSnapportSize.height) &&
calcSnapPoints.YDistanceBetweenBestAndSecondEdge() >
aSnapInfo.mSnapportSize.height) {
calcSnapPoints.AddHorizontalEdge(CalcSnapPoints::SnapPosition{
clampedDestination.y, StyleScrollSnapStop::Normal, range.mTargetId});
break;
}
}
bool snapped = false;
auto finalPos = calcSnapPoints.GetBestEdge();
nscoord proximityThreshold =
StaticPrefs::layout_css_scroll_snap_proximity_threshold();
proximityThreshold = nsPresContext::CSSPixelsToAppUnits(proximityThreshold);
if (aSnapInfo.mScrollSnapStrictnessY ==
StyleScrollSnapStrictness::Proximity &&
std::abs(aDestination.y - finalPos.mPosition.y) > proximityThreshold) {
finalPos.mPosition.y = aDestination.y;
} else if (aSnapInfo.mScrollSnapStrictnessY !=
StyleScrollSnapStrictness::None &&
aDestination.y != finalPos.mPosition.y) {
snapped = true;
}
if (aSnapInfo.mScrollSnapStrictnessX ==
StyleScrollSnapStrictness::Proximity &&
std::abs(aDestination.x - finalPos.mPosition.x) > proximityThreshold) {
finalPos.mPosition.x = aDestination.x;
} else if (aSnapInfo.mScrollSnapStrictnessX !=
StyleScrollSnapStrictness::None &&
aDestination.x != finalPos.mPosition.x) {
snapped = true;
}
return snapped ? Some(finalPos) : Nothing();
}
ScrollSnapTargetId ScrollSnapUtils::GetTargetIdFor(const nsIFrame* aFrame) {
MOZ_ASSERT(aFrame && aFrame->GetContent());
return ScrollSnapTargetId{reinterpret_cast<uintptr_t>(aFrame->GetContent())};
}
static std::pair<Maybe<nscoord>, Maybe<nscoord>> GetCandidateInLastTargets(
const layers::ScrollSnapInfo& aSnapInfo, const nsPoint& aCurrentPosition,
const UniquePtr<ScrollSnapTargetIds>& aLastSnapTargetIds,
const nsIContent* aFocusedContent) {
ScrollSnapTargetId targetIdForFocusedContent = ScrollSnapTargetId::None;
if (aFocusedContent && aFocusedContent->GetPrimaryFrame()) {
targetIdForFocusedContent =
ScrollSnapUtils::GetTargetIdFor(aFocusedContent->GetPrimaryFrame());
}
// Note: Below algorithm doesn't care about cases where the last snap point
// was on an element larger than the snapport since it's not clear to us
// what we should do for now.
// https://github.com/w3c/csswg-drafts/issues/7438
const ScrollSnapInfo::SnapTarget* focusedTarget = nullptr;
Maybe<nscoord> x, y;
for (const auto& target : aSnapInfo.mSnapTargets) {
if (!target.IsValidFor(aCurrentPosition, aSnapInfo.mSnapportSize)) {
continue;
}
if (target.mSnapPositionX &&
aSnapInfo.mScrollSnapStrictnessX != StyleScrollSnapStrictness::None) {
if (aLastSnapTargetIds->mIdsOnX.Contains(target.mTargetId)) {
if (targetIdForFocusedContent == target.mTargetId) {
// If we've already found the candidate on Y axis, but if snapping to
// the point results this target is scrolled out, we can't use it.
if ((y && !target.mSnapArea.Intersects(
nsRect(nsPoint(*target.mSnapPositionX, *y),
aSnapInfo.mSnapportSize)))) {
y.reset();
}
focusedTarget = ⌖
// If the focused one is valid, then it's the candidate.
x = target.mSnapPositionX;
}
if (!x) {
// Update the candidate on X axis only if
// 1) we haven't yet found the candidate on Y axis
// 2) or if we've found the candiate on Y axis and if snapping to the
// candidate position result the target element is visible inside
// the snapport.
if (!y || (y && target.mSnapArea.Intersects(
nsRect(nsPoint(*target.mSnapPositionX, *y),
aSnapInfo.mSnapportSize)))) {
x = target.mSnapPositionX;
}
}
}
}
if (target.mSnapPositionY &&
aSnapInfo.mScrollSnapStrictnessY != StyleScrollSnapStrictness::None) {
if (aLastSnapTargetIds->mIdsOnY.Contains(target.mTargetId)) {
if (targetIdForFocusedContent == target.mTargetId) {
NS_ASSERTION(!focusedTarget || focusedTarget == &target,
"If the focused target has been found on X axis, the "
"target should be same");
// If we've already found the candidate on X axis other than the
// focused one, but if snapping to the point results this target is
// scrolled out, we can't use it.
if (!focusedTarget && (x && !target.mSnapArea.Intersects(nsRect(
nsPoint(*x, *target.mSnapPositionY),
aSnapInfo.mSnapportSize)))) {
x.reset();
}
focusedTarget = ⌖
y = target.mSnapPositionY;
}
if (!y) {
if (!x || (x && target.mSnapArea.Intersects(
nsRect(nsPoint(*x, *target.mSnapPositionY),
aSnapInfo.mSnapportSize)))) {
y = target.mSnapPositionY;
}
}
}
}
// If we found candidates on both axes, it's the one we need.
if (x && y &&
// If we haven't found the focused target, it's possible that we haven't
// iterated it, don't break in such case.
(targetIdForFocusedContent == ScrollSnapTargetId::None ||
focusedTarget)) {
break;
}
}
return {x, y};
}
Maybe<mozilla::SnapTarget> ScrollSnapUtils::GetSnapPointForResnap(
const layers::ScrollSnapInfo& aSnapInfo, const nsRect& aScrollRange,
const nsPoint& aCurrentPosition,
const UniquePtr<ScrollSnapTargetIds>& aLastSnapTargetIds,
const nsIContent* aFocusedContent) {
if (!aLastSnapTargetIds) {
return GetSnapPointForDestination(aSnapInfo, ScrollUnit::DEVICE_PIXELS,
ScrollSnapFlags::IntendedEndPosition,
aScrollRange, aCurrentPosition,
aCurrentPosition);
}
auto [x, y] = GetCandidateInLastTargets(aSnapInfo, aCurrentPosition,
aLastSnapTargetIds, aFocusedContent);
if (!x && !y) {
// In the worst case there's no longer valid snap points previously snapped,
// try to find new valid snap points.
return GetSnapPointForDestination(aSnapInfo, ScrollUnit::DEVICE_PIXELS,
ScrollSnapFlags::IntendedEndPosition,
aScrollRange, aCurrentPosition,
aCurrentPosition);
}
// If there's no candidate on one of the axes in the last snap points, try
// to find a new candidate.
if (!x || !y) {
nsPoint newPosition =
nsPoint(x ? *x : aCurrentPosition.x, y ? *y : aCurrentPosition.y);
CalcSnapPoints calcSnapPoints(ScrollUnit::DEVICE_PIXELS,
ScrollSnapFlags::IntendedEndPosition,
newPosition, newPosition);
for (const auto& target : aSnapInfo.mSnapTargets) {
if (!target.IsValidFor(newPosition, aSnapInfo.mSnapportSize)) {
continue;
}
if (!x && target.mSnapPositionX &&
aSnapInfo.mScrollSnapStrictnessX != StyleScrollSnapStrictness::None) {
calcSnapPoints.AddVerticalEdge(
{*target.mSnapPositionX, target.mScrollSnapStop, target.mTargetId});
}
if (!y && target.mSnapPositionY &&
aSnapInfo.mScrollSnapStrictnessY != StyleScrollSnapStrictness::None) {
calcSnapPoints.AddHorizontalEdge(
{*target.mSnapPositionY, target.mScrollSnapStop, target.mTargetId});
}
}
auto finalPos = calcSnapPoints.GetBestEdge();
if (!x) {
x = Some(finalPos.mPosition.x);
}
if (!y) {
y = Some(finalPos.mPosition.y);
}
}
SnapTarget snapTarget{nsPoint(*x, *y)};
// Collect snap points where the position is still same as the new snap
// position.
for (const auto& target : aSnapInfo.mSnapTargets) {
if (!target.IsValidFor(snapTarget.mPosition, aSnapInfo.mSnapportSize)) {
continue;
}
if (target.mSnapPositionX &&
aSnapInfo.mScrollSnapStrictnessX != StyleScrollSnapStrictness::None &&
target.mSnapPositionX == x) {
snapTarget.mTargetIds.mIdsOnX.AppendElement(target.mTargetId);
}
if (target.mSnapPositionY &&
aSnapInfo.mScrollSnapStrictnessY != StyleScrollSnapStrictness::None &&
target.mSnapPositionY == y) {
snapTarget.mTargetIds.mIdsOnY.AppendElement(target.mTargetId);
}
}
return Some(snapTarget);
}
void ScrollSnapUtils::PostPendingResnapIfNeededFor(nsIFrame* aFrame) {
ScrollSnapTargetId id = GetTargetIdFor(aFrame);
if (id == ScrollSnapTargetId::None) {
return;
}
if (nsIScrollableFrame* sf = nsLayoutUtils::GetNearestScrollableFrame(
aFrame, nsLayoutUtils::SCROLLABLE_SAME_DOC |
nsLayoutUtils::SCROLLABLE_INCLUDE_HIDDEN)) {
sf->PostPendingResnapIfNeeded(aFrame);
}
}
void ScrollSnapUtils::PostPendingResnapFor(nsIFrame* aFrame) {
if (nsIScrollableFrame* sf = nsLayoutUtils::GetNearestScrollableFrame(
aFrame, nsLayoutUtils::SCROLLABLE_SAME_DOC |
nsLayoutUtils::SCROLLABLE_INCLUDE_HIDDEN)) {
sf->PostPendingResnap();
}
}
bool ScrollSnapUtils::NeedsToRespectTargetWritingMode(
const nsSize& aSnapAreaSize, const nsSize& aSnapportSize) {
// Use the writing-mode on the target element if the snap area is larger than
// the snapport.
// https://drafts.csswg.org/css-scroll-snap/#snap-scope
//
// It's unclear `larger` means that the size is larger than only on the target
// axis. If it doesn't, it will pick the same axis in the case where only one
// axis is larger. For example, if an element size is (200 x 10) and the
// snapport size is (100 x 100) and if the element's writing mode is different
// from the scroller's writing mode, then `scroll-snap-align: start start`
// will be conflict.
return aSnapAreaSize.width > aSnapportSize.width ||
aSnapAreaSize.height > aSnapportSize.height;
}
static nsRect InflateByScrollMargin(const nsRect& aTargetRect,
const nsMargin& aScrollMargin,
const nsRect& aScrolledRect) {
// Inflate the rect by scroll-margin.
nsRect result = aTargetRect;
result.Inflate(aScrollMargin);
// But don't be beyond the limit boundary.
return result.Intersect(aScrolledRect);
}
nsRect ScrollSnapUtils::GetSnapAreaFor(const nsIFrame* aFrame,
const nsIFrame* aScrolledFrame,
const nsRect& aScrolledRect) {
nsRect targetRect = nsLayoutUtils::TransformFrameRectToAncestor(
aFrame, aFrame->GetRectRelativeToSelf(), aScrolledFrame);
// The snap area contains scroll-margin values.
// https://drafts.csswg.org/css-scroll-snap-1/#scroll-snap-area
nsMargin scrollMargin = aFrame->StyleMargin()->GetScrollMargin();
return InflateByScrollMargin(targetRect, scrollMargin, aScrolledRect);
}
} // namespace mozilla
