z-bar-qt/Plugins/ZShell/Components/snaplayout.cpp

#include "snaplayout.hpp"

#include <QLoggingCategory>
#include <QQuickWindow>
#include <QScopeGuard>
#include <QtGui/private/qfont_p.h>
#include <algorithm>
#include <cmath>

Q_LOGGING_CATEGORY(lcQuickLayouts, "qt.quick.layouts")

namespace ZShell::components {
namespace {

qint64 originResidual(qreal logical, qreal dpr) {
	const qreal device = logical * dpr;
	return std::llround((device - std::floor(device)) * 1024.0) % 1024;
}

// Snap one size hint to the device grid. `mode` is >0 to round up, <0 to round
// down, 0 to round to nearest. Infinities and NaN are left alone: they mean
// "unbounded".
qreal snapSizeHint(qreal value, qreal dpr, int mode) {
	if (qIsNaN(value) || std::isinf(value)) return value;

	const double exact = value * dpr;
	const double snapped = mode > 0 ? std::ceil(exact - 1e-9)
						   : mode < 0
							   ? std::floor(exact + 1e-9)
							   : static_cast<double>(std::llround(exact));
	return snapped / dpr;
}

QPointF absoluteOrigin(const QQuickItem* item) {
	qreal x = 0.0;
	qreal y = 0.0;
	for (const QQuickItem* it = item; it; it = it->parentItem()) {
		x += it->x();
		y += it->y();
	}
	return QPointF(x, y);
}

} // namespace

qreal SnapLayoutStyleInfo::spacing(Qt::Orientation /*orientation*/) const {
#if defined(Q_OS_ANDROID) || defined(Q_OS_IOS) || defined(Q_OS_QNX)
	qreal spacing = 8.0;
#else
	qreal spacing = 5.0;
#endif
#if !defined(Q_OS_MACOS) && !defined(Q_OS_HARMONY)
	spacing = qRound(spacing * (qreal(qt_defaultDpiX()) / 96.0));
#endif
	return spacing;
}

qreal SnapLayoutStyleInfo::windowMargin(Qt::Orientation /*orientation*/) const {
	return 0;
}

bool SnapLayoutStyleInfo::hasChangedCore() const {
	return false;
}

SnapGridLayoutItem::SnapGridLayoutItem(
	QQuickItem* item,
	int row,
	int column,
	int rowSpan,
	int columnSpan,
	Qt::Alignment alignment)
	: QQuickGridLayoutItem(item, row, column, rowSpan, columnSpan, alignment) {}

void SnapGridLayoutItem::setGeometry(const QRectF& rect) {
	QQuickLayoutAttached* info = attachedLayoutObject(m_item, false);
	engineRect = info ? rect.marginsRemoved(info->effectiveQMargins()) : rect;
	placed = true;
}

void SnapLayoutPrivate::mirrorChange() {
	Q_Q(SnapLayout);
	q->invalidate();
}

SnapLayout::SnapLayout(QQuickItem* parent)
	: QQuickLayout(*new SnapLayoutPrivate, parent) {
	d_func()->styleInfo = new SnapLayoutStyleInfo;
}

SnapLayout::~SnapLayout() {
	Q_D(SnapLayout);
	clearAncestorListeners();
	deactivateRecur();
	delete d->styleInfo;
}

void SnapLayout::componentComplete() {
	QQuickLayout::componentComplete();

	applySpacing();
	invalidate();
	ensureLayoutItemsUpdated(QQuickLayout::ApplySizeHints);

	if (qobject_cast<QQuickLayout*>(parentItem())) return;
	rearrange(QSizeF(width(), height()));
}

void SnapLayout::itemChange(ItemChange change, const ItemChangeData& value) {
	QQuickLayout::itemChange(change, value);
	if (change == ItemChildRemovedChange && value.item)
		d_func()->childSizes.remove(value.item);
	if (change == ItemDevicePixelRatioHasChanged || change == ItemSceneChange) {
		applySpacing();
		invalidate();
	}
}

void SnapLayout::geometryChange(
	const QRectF& newGeometry, const QRectF& oldGeometry) {
	Q_D(SnapLayout);
	QQuickLayout::geometryChange(newGeometry, oldGeometry);

	if (!isReady()) return;

	const QQuickWindow* win = window();
	const qreal dpr = win ? win->effectiveDevicePixelRatio() : 0.0;
	if (dpr <= 0.0) return;

	// Any change of our own geometry can move us: our size changed and an anchor
	// on a bottom or right edge moved us with it, or an ancestor resized and the
	// same anchors moved us. Children were placed relative to the origin the
	// arrangement saw: their local positions carry the compensation for that
	// origin's fraction of a device pixel, so after a move they land on a
	// fraction - which is why a centered child is blurred by a different amount
	// every time the panel opens. Re-arrange whenever the origin's residual
	// changes; it is remembered on every rearrange, so this fires only when a
	// re-arrange is really needed, and it converges as soon as the animation
	// stops moving the layout.
	const QPointF origin = absoluteOrigin(this);
	if (originResidual(origin.x(), dpr) == d->originResidualX &&
		originResidual(origin.y(), dpr) == d->originResidualY)
		return;

	rearrangeForOriginMove();
}

// Children are placed against the origin the arrangement saw, so a move that
// changes the origin's fraction of a device pixel needs a new arrangement.
// polish() defers that to the next frame, and a polish request does nothing
// while one is already pending - if no frame follows, or the pending polish is
// never delivered, the children stay placed against the old origin forever.
// Rearrange immediately instead; polish() is only the fallback for the cases
// where rearranging now is not possible.
void SnapLayout::rearrangeForOriginMove() {
	Q_D(SnapLayout);
	const QSizeF size(width(), height());
	if (d->rearranging || size.width() <= 0.0 || size.height() <= 0.0) {
		polish();
		return;
	}
	rearrange(size);
}

QSizeF SnapLayout::sizeHint(Qt::SizeHint whichSizeHint) const {
	Q_D(const SnapLayout);
	const QSizeF engineHint =
		d->engine.sizeHint(whichSizeHint, QSizeF(), d->styleInfo);

	const QQuickWindow* win = window();
	const qreal dpr = win ? win->effectiveDevicePixelRatio() : 1.0;
	if (dpr <= 0.0) return engineHint;

	const bool horiz = d->orientation == Qt::Horizontal;
	// Which way the children are traversed, so the leading and trailing margin
	// of a child are the sides the arrangement measures its gap from.
	const int sgn = horiz && effectiveLayoutDirection() == Qt::RightToLeft ? -1
																		   : 1;

	// Qt builds implicit sizes out of whole logical pixels:
	// QQuickLayout::effectiveSizeHints_helper takes `qCeil(item->implicitHeight())`
	// for every child, so the sum can be an odd number of logical pixels, which is
	// a half device pixel at a fractional ratio - 37 logical pixels at 1.5x is 55.5
	// device pixels. A layout that sizes itself from its implicit size then has an
	// edge on a fraction of a device pixel, and anchored to a bottom edge its top
	// edge - and every child measured down from that top - lands off the grid.
	//
	// The preferred size is therefore summed here in device pixels: each child's
	// own hint rounded up to the device grid, plus its Layout margins and the
	// device-normalized spacing. That is exactly what the arrangement writes, so
	// the layout is neither larger than its children (dead space, and a Flickable
	// that reports more content height than the content occupies) nor smaller
	// (the last child cut off).
	qint64 mainExtentDev = 0;
	qint64 crossDev = 0;
	qint64 maxMainDev = 0;
	int count = 0;

	for (int i = 0; i < d->engine.itemCount(); ++i) {
		const SnapGridLayoutItem* gridItem =
			static_cast<const SnapGridLayoutItem*>(d->engine.itemAt(i));
		QQuickItem* child = gridItem->layoutItem();
		if (!child || shouldIgnoreItem(child)) continue;

		QQuickLayoutAttached* info = attachedLayoutObject(child, false);
		const QMarginsF margins = info ? info->effectiveQMargins()
									   : QMarginsF(0, 0, 0, 0);

		// The engine's per-child hints are authoritative: they follow
		// Layout.preferredWidth/Height when those are set, otherwise the child's
		// implicit size, otherwise the first width and height it had, and they
		// include the child's Layout margins. They are also cached, so a nested
		// layout contributes its own preferred size even before it is polished.
		const QSizeF* hints = gridItem->effectiveSizeHints();

		const qreal mainMargin = horiz ? margins.left() + margins.right()
									   : margins.top() + margins.bottom();
		const qreal crossMargin = horiz ? margins.top() + margins.bottom()
										: margins.left() + margins.right();

		qreal mainHint = (horiz ? hints[Qt::PreferredSize].width()
								: hints[Qt::PreferredSize].height()) -
						 mainMargin;
		qreal crossHint = (horiz ? hints[Qt::PreferredSize].height()
								 : hints[Qt::PreferredSize].width()) -
						  crossMargin;

		// The engine hint is `qCeil` of the implicit size, so it can sit up to a
		// whole logical pixel above it - a device pixel per child at 1.5x, which
		// adds up to dead space in a long column. When the child's own implicit
		// size is known and the hint is just that ceiling, measure the implicit
		// size and let the device rounding be the only rounding.
		const qreal ownMain = horiz ? child->implicitWidth()
									: child->implicitHeight();
		if (ownMain > 0.0 && mainHint <= ownMain + 1.0) mainHint = ownMain;

		const qreal ownCross = horiz ? child->implicitHeight()
									 : child->implicitWidth();
		if (ownCross > 0.0 && crossHint <= ownCross + 1.0) crossHint = ownCross;

		if (!std::isfinite(mainHint) || mainHint < 0.0) mainHint = 0.0;
		if (!std::isfinite(crossHint) || crossHint < 0.0) crossHint = 0.0;

		// Rounding up keeps the hint from ever under-sizing its contents.
		const qint64 mainDev = qint64(std::ceil(mainHint * dpr - 1e-9));
		// Every margin side is rounded on its own, exactly as the arrangement
		// rounds it when it places the child. Rounding the two sides of a child
		// together can leave the hint a device pixel short of what is written -
		// two one-pixel margins are three device pixels at 1.5x when summed but
		// four when rounded separately - and a Flickable that takes this hint as
		// its content height would then have bounds that cut into the content.
		const qreal mainLead =
			horiz ? (sgn > 0 ? margins.left() : margins.right())
				  : margins.top();
		const qreal mainTrail =
			horiz ? (sgn > 0 ? margins.right() : margins.left())
				  : margins.bottom();
		const qint64 mainMarginDev =
			std::llround(mainLead * dpr) + std::llround(mainTrail * dpr);
		const qreal crossLead =
			horiz ? margins.top()
				  : (sgn > 0 ? margins.left() : margins.right());
		const qreal crossTrail =
			horiz ? margins.bottom()
				  : (sgn > 0 ? margins.right() : margins.left());
		const qint64 crossDevForChild =
			qint64(std::ceil(crossHint * dpr - 1e-9)) +
			std::llround(crossLead * dpr) + std::llround(crossTrail * dpr);

		mainExtentDev += mainDev + mainMarginDev;
		maxMainDev = (std::max)(maxMainDev, mainDev + mainMarginDev);
		crossDev = (std::max)(crossDev, crossDevForChild);
		++count;
	}

	qreal space = d->spacingRequest;
	if (qIsNaN(space)) space = d->styleInfo->spacing(d->orientation);
	const qint64 gapDev = dpr > 0.0 ? qint64(std::llround(space * dpr))
									: qint64(space);

	if (uniformCellSizes() && count > 0)
		mainExtentDev = maxMainDev * qint64(count);
	if (count > 1) mainExtentDev += gapDev * qint64(count - 1);

	const QSizeF preferred(
		horiz ? qreal(mainExtentDev) / dpr : qreal(crossDev) / dpr,
		horiz ? qreal(crossDev) / dpr : qreal(mainExtentDev) / dpr);

	if (whichSizeHint == Qt::PreferredSize) return preferred;

	if (whichSizeHint == Qt::MinimumSize) {
		// Minimums round up, but never past the preferred size, or cells would
		// grow past what the arrangement writes.
		const QSizeF minimum(
			snapSizeHint(engineHint.width(), dpr, 1),
			snapSizeHint(engineHint.height(), dpr, 1));
		return QSizeF(
			(std::min)(minimum.width(), preferred.width()),
			(std::min)(minimum.height(), preferred.height()));
	}

	// Maximums round down, but never below the preferred size: a layout is always
	// allowed to be as large as its contents need.
	const QSizeF maximum(
		snapSizeHint(engineHint.width(), dpr, -1),
		snapSizeHint(engineHint.height(), dpr, -1));
	return QSizeF(
		(std::max)(maximum.width(), preferred.width()),
		(std::max)(maximum.height(), preferred.height()));
}

Qt::Orientation SnapLayout::orientation() const {
	return d_func()->orientation;
}

void SnapLayout::setOrientation(Qt::Orientation orientation) {
	Q_D(SnapLayout);
	if (d->orientation == orientation) return;
	d->orientation = orientation;
	invalidate();
	emit orientationChanged();
}

Qt::LayoutDirection SnapLayout::layoutDirection() const {
	return d_func()->layoutDirection;
}

void SnapLayout::setLayoutDirection(Qt::LayoutDirection direction) {
	Q_D(SnapLayout);
	if (d->layoutDirection == direction) return;
	d->layoutDirection = direction;
	invalidate();
	emit layoutDirectionChanged();
}

Qt::LayoutDirection SnapLayout::effectiveLayoutDirection() const {
	return !isMirrored() == (layoutDirection() == Qt::LeftToRight)
			   ? Qt::LeftToRight
			   : Qt::RightToLeft;
}

qreal SnapLayout::spacing() const {
	Q_D(const SnapLayout);
	if (!std::isnan(d->spacingRequest)) return d->spacingRequest;
	return d->styleInfo->spacing(d->orientation);
}

void SnapLayout::applySpacing() {
	Q_D(SnapLayout);
	if (!isReady()) return;

	const QQuickWindow* win = window();
	const qreal dpr = win ? win->effectiveDevicePixelRatio() : 1.0;

	qreal space = d->spacingRequest;
	if (std::isnan(space)) space = d->styleInfo->spacing(d->orientation);

	if (dpr > 0.0) space = qreal(std::llround(space * dpr)) / dpr;

	d->engine.setSpacing(space, Qt::Horizontal | Qt::Vertical);
}

void SnapLayout::setSpacing(qreal space) {
	Q_D(SnapLayout);
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wfloat-equal"
	if (std::isnan(space) || d->spacingRequest == space) return;
#pragma GCC diagnostic pop
	d->spacingRequest = space;
	applySpacing();
	invalidate();
	emit spacingChanged();
}

bool SnapLayout::uniformCellSizes() const {
	Q_D(const SnapLayout);
	return d->engine.uniformCellWidths();
}

void SnapLayout::setUniformCellSizes(bool uniformCellSizes) {
	Q_D(SnapLayout);
	if (d->engine.uniformCellHeights() == uniformCellSizes) return;
	d->engine.setUniformCellWidths(uniformCellSizes);
	d->engine.setUniformCellHeights(uniformCellSizes);
	invalidate();
	emit uniformCellSizesChanged();
}

void SnapLayout::setAlignment(QQuickItem* item, Qt::Alignment align) {
	Q_D(SnapLayout);
	if (QQuickGridLayoutItem* layoutItem = d->engine.findLayoutItem(item)) {
		layoutItem->setAlignment(align);
		d->engine.invalidate();
	}
	maybeSubscribeToBaseLineOffsetChanges(item);
}

void SnapLayout::setStretchFactor(
	QQuickItem* item, int stretchFactor, Qt::Orientation orient) {
	Q_D(SnapLayout);
	if (QQuickGridLayoutItem* layoutItem = d->engine.findLayoutItem(item)) {
		layoutItem->setStretchFactor(stretchFactor, orient);
		d->engine.invalidate();
	}
}

void SnapLayout::invalidate(QQuickItem* childItem) {
	Q_D(SnapLayout);
	if (!isReady()) return;

	if (childItem) {
		if (d->rearranging) {
			if (!d->invalidateAfterRearrange.contains(childItem))
				d->invalidateAfterRearrange << childItem;
			return;
		}
		if (QQuickGridLayoutItem* layoutItem =
				d->engine.findLayoutItem(childItem)) {
			layoutItem->invalidate();
		}
	}

	d->engine.invalidate();
	QQuickLayout::invalidate();

	if (auto* parentLayout = qobject_cast<QQuickLayout*>(parentItem()))
		parentLayout->invalidate(this);
}

void SnapLayout::updateLayoutItems() {
	Q_D(SnapLayout);
	if (!isReady()) return;

	const auto items = childItems();
	for (auto entry = d->childSizes.begin(); entry != d->childSizes.end();) {
		if (items.contains(entry.key()))
			++entry;
		else
			entry = d->childSizes.erase(entry);
	}

	d->engine.deleteItems();
	insertLayoutItems();
}

QQuickItem* SnapLayout::itemAt(int index) const {
	Q_D(const SnapLayout);
	return static_cast<QQuickGridLayoutItem*>(d->engine.itemAt(index))
		->layoutItem();
}

int SnapLayout::itemCount() const {
	Q_D(const SnapLayout);
	return d->engine.itemCount();
}

void SnapLayout::insertLayoutItems() {
	Q_D(SnapLayout);
	const auto items = childItems();
	for (QQuickItem* child : items) {
		Q_ASSERT(child);
		checkAnchors(child);

		if (shouldIgnoreItem(child)) continue;
		QQuickLayoutAttached* info = attachedLayoutObject(child, false);

		Qt::Alignment alignment;
		int hStretch = -1;
		int vStretch = -1;
		bool fillWidth = false;
		bool fillHeight = false;
		if (info) {
			alignment = info->alignment();
			hStretch = info->horizontalStretchFactor();
			vStretch = info->verticalStretchFactor();
			fillWidth = info->fillWidth();
			fillHeight = info->fillHeight();
		}

		const int index = d->engine.rowCount(d->orientation);
		d->engine.insertRow(index, d->orientation);

		int gridRow = 0;
		int gridColumn = index;
		if (d->orientation == Qt::Vertical) qSwap(gridRow, gridColumn);
		QGridLayoutItem* layoutItem =
			new SnapGridLayoutItem(child, gridRow, gridColumn, 1, 1, alignment);

		if (hStretch >= 0) {
			if (!fillWidth)
				qWarning() << "horizontalStretchFactor requires fillWidth to "
							  "also be set to true";
			layoutItem->setStretchFactor(hStretch, Qt::Horizontal);
		}
		if (vStretch >= 0) {
			if (!fillHeight)
				qWarning() << "verticalStretchFactor requires fillHeight to "
							  "also be set to true";
			layoutItem->setStretchFactor(vStretch, Qt::Vertical);
		}
		d->engine.insertItem(layoutItem, index);
	}
}

void SnapLayout::removeGridItem(QGridLayoutItem* gridItem) {
	Q_D(SnapLayout);
	const int index = gridItem->firstRow(d->orientation);
	d->engine.removeItem(gridItem);
	d->engine.removeRows(index, 1, d->orientation);
}

void SnapLayout::itemDestroyed(QQuickItem* item) {
	if (!isReady()) return;
	Q_D(SnapLayout);
	d->childSizes.remove(item);
	if (QQuickGridLayoutItem* gridItem = d->engine.findLayoutItem(item)) {
		removeGridItem(gridItem);
		delete gridItem;
		invalidate();
	}
}

void SnapLayout::itemVisibilityChanged(QQuickItem* item) {
	if (!isReady()) return;
	invalidate(item);
}

void SnapLayout::itemGeometryChanged(
	QQuickItem* item, QQuickGeometryChange change, const QRectF& oldGeometry) {
	Q_UNUSED(item)
	Q_UNUSED(change)
	Q_UNUSED(oldGeometry)
	if (!isReady()) return;

	const QQuickWindow* win = window();
	const qreal dpr = win ? win->effectiveDevicePixelRatio() : 0.0;
	if (dpr <= 0.0) return;

	// An ancestor changing size can move us too - a layout anchored to a bottom
	// edge slides when its parent grows - so only the origin's fraction of a
	// device pixel matters, not which kind of change reached us.
	Q_D(SnapLayout);
	const QPointF origin = absoluteOrigin(this);
	if (originResidual(origin.x(), dpr) == d->originResidualX &&
		originResidual(origin.y(), dpr) == d->originResidualY)
		return;

	rearrangeForOriginMove();
}

void SnapLayout::updateAncestorListeners() {
	Q_D(SnapLayout);

	const QFlags<QQuickItemPrivate::ChangeType> mask =
		QQuickItemPrivate::Geometry;

	QVector<QQuickItem*> current;
	for (QQuickItem* it = parentItem(); it; it = it->parentItem()) {
		if (qobject_cast<SnapLayout*>(it)) break;
		current.append(it);
	}

	for (const QPointer<QQuickItem>& ancestor :
		 std::as_const(d->ancestorListeners)) {
		if (ancestor && !current.contains(ancestor.data())) {
			QQuickItemPrivate::get(ancestor.data())
				->removeItemChangeListener(
					this, d->ancestorMasks.value(ancestor.data()));
			d->ancestorMasks.remove(ancestor.data());
		}
	}

	// Qt appends one registration per addItemChangeListener call, so only the
	// bits that are not registered yet may be added; re-adding the same mask on
	// every rearrange would duplicate listeners without bound.
	for (QQuickItem* ancestor : std::as_const(current)) {
		const auto have = d->ancestorMasks.value(ancestor);
		const auto missing = mask & ~have;
		const auto extra = have & ~mask;

		if (extra != 0) {
			QQuickItemPrivate::get(ancestor)->removeItemChangeListener(
				this, extra);
		}
		if (missing != 0) {
			QQuickItemPrivate::get(ancestor)->addItemChangeListener(
				this, missing);
		}

		d->ancestorMasks[ancestor] = mask;
	}

	d->ancestorListeners.clear();
	d->ancestorListeners.reserve(current.size());
	for (QQuickItem* ancestor : std::as_const(current))
		d->ancestorListeners.append(ancestor);
}

void SnapLayout::clearAncestorListeners() {
	Q_D(SnapLayout);
	for (const QPointer<QQuickItem>& ancestor :
		 std::as_const(d->ancestorListeners)) {
		if (ancestor) {
			QQuickItemPrivate::get(ancestor.data())
				->removeItemChangeListener(
					this, d->ancestorMasks.value(ancestor.data()));
		}
	}
	d->ancestorMasks.clear();
	d->ancestorListeners.clear();
}

void SnapLayout::arrangeChildren() {
	Q_D(SnapLayout);
	const QQuickWindow* win = window();
	const qreal dpr = win ? win->effectiveDevicePixelRatio() : 1.0;
	if (dpr <= 0.0) return;

	const bool horiz = d->orientation == Qt::Horizontal;
	const QPointF origin = absoluteOrigin(this);
	const qint64 originXDev = std::llround(origin.x() * dpr);
	const qint64 originYDev = std::llround(origin.y() * dpr);

	// The layout's own main extent, rounded down: no child is placed past it, so
	// a Flickable that takes this layout's height as its content height can still
	// reach the last child.
	const QSizeF self = size();
	const qint64 layoutMainExtentDev =
		qint64(std::floor((horiz ? self.width() : self.height()) * dpr + 1e-6));

	// An aligned child whose cell is its own implicit size is sized from that
	// implicit size. The engine recomputes cells from animated geometry, so a
	// cell that is mathematically 41 can arrive as 40.9999999999; when the
	// device pixel ratio puts 41 exactly on half a device pixel, that epsilon
	// flips the rounding from pass to pass and the child visibly resizes while
	// it only moves. Children that fill their axis, or whose cell has grown
	// past their implicit size, keep the engine-derived size so that animated
	// fills still step through device pixels.
	const auto snappedOwn = [&](bool fills, qreal cellSize, qreal ownSize) {
		if (fills || ownSize <= 0.0 || qAbs(cellSize - ownSize) >= 1.0)
			return qint64(-1);
		// Round up, like Qt's `qCeil` of implicit sizes: a child is never
		// smaller than the content it holds, and this is the same rounding
		// sizeHint() sums to compute the layout's own preferred size, so the
		// children fill the layout without slack and without overflow.
		return (std::max)(qint64(std::ceil(ownSize * dpr - 1e-9)), qint64(0));
	};

	struct Row {
		QQuickItem* child = nullptr;
		qreal mainExact = 0.0;	   // engine start, device pixels
		qreal mainSizeExact = 0.0; // engine main size, device pixels
		qint64 mainFarExact = 0;   // engine end, device pixels
		qint64 crossExact = 0;	   // engine cross start, device pixels
		qint64 engineWidth = 0;
		qint64 engineHeight = 0;
		qint64 mainOwnDev = -1;
		qint64 crossOwnDev = -1;
		qint64 marginLeadDev =
			0; // Layout margin before the child, device pixels
		qint64 marginTrailDev =
			0; // Layout margin after the child, device pixels
		bool fillsMain = false;
		qint64 pos = 0;	  // leading edge in traversal device pixels
		qint64 size = 0;  // main size to write
		qint64 cross = 0; // cross size to write
	};

	// Traversal coordinates run along the main axis in child order: left to right
	// unless the layout is mirrored. A child's `pos` is its leading edge in that
	// direction, so it spans [pos, pos + size] whichever way the layout runs, and
	// the chain and the anchoring below are direction-free.
	const int sgn = horiz && effectiveLayoutDirection() == Qt::RightToLeft ? -1
																		   : 1;

	// Spacing is normalized to whole device pixels, and each child's Layout
	// margins are rounded on their own rather than together with the spacing:
	// rounding the sum makes the same spacing read as 7 or 8 device pixels
	// depending on which two children it sits between.
	const qint64 spacingDev = std::llround(spacing() * dpr);

	QVector<Row> rows;
	rows.reserve(d->engine.itemCount());

	for (int i = 0; i < d->engine.itemCount(); ++i) {
		SnapGridLayoutItem* gridItem =
			static_cast<SnapGridLayoutItem*>(d->engine.itemAt(i));
		if (!gridItem->placed) continue;
		QQuickItem* child = gridItem->layoutItem();
		const QRectF& cell = gridItem->engineRect;

		const qreal mainExact = (horiz ? cell.x() : cell.y()) * dpr;
		const qreal mainSizeExact =
			(horiz ? cell.width() : cell.height()) * dpr;

		Row row;
		row.child = child;
		row.mainExact = mainExact;
		row.mainSizeExact = mainSizeExact;
		row.mainFarExact = std::llround(mainExact + mainSizeExact);
		row.crossExact = std::llround((horiz ? cell.y() : cell.x()) * dpr);

		const qint64 mainSizeDev = std::llround(mainSizeExact);
		const qint64 crossSizeDev =
			std::llround((horiz ? cell.height() : cell.width()) * dpr);
		row.engineWidth = horiz ? mainSizeDev : crossSizeDev;
		row.engineHeight = horiz ? crossSizeDev : mainSizeDev;

		QQuickLayoutAttached* info = attachedLayoutObject(child, false);
		const qreal mainOwn = horiz ? child->implicitWidth()
									: child->implicitHeight();
		const qreal crossOwn = horiz ? child->implicitHeight()
									 : child->implicitWidth();
		row.fillsMain = info &&
						(horiz ? info->fillWidth() : info->fillHeight());
		const bool fillsCross =
			info && (horiz ? info->fillHeight() : info->fillWidth());
		row.mainOwnDev = snappedOwn(
			row.fillsMain, horiz ? cell.width() : cell.height(), mainOwn);
		row.crossOwnDev = snappedOwn(
			fillsCross, horiz ? cell.height() : cell.width(), crossOwn);

		const QMarginsF margins = info ? info->effectiveQMargins()
									   : QMarginsF(0, 0, 0, 0);
		const qreal marginLead =
			horiz ? (sgn > 0 ? margins.left() : margins.right())
				  : margins.top();
		const qreal marginTrail =
			horiz ? (sgn > 0 ? margins.right() : margins.left())
				  : margins.bottom();
		row.marginLeadDev = std::llround(marginLead * dpr);
		row.marginTrailDev = std::llround(marginTrail * dpr);
		rows.append(row);
	}

	if (rows.isEmpty()) return;

	// The layout's own edges in traversal coordinates. The first child is placed
	// against the start edge and the last one back from the end edge, each by its
	// own rounded margin - the same rounding the gaps and `sizeHint` use, so the
	// layout's implicit size is the size its children actually occupy. Rounding
	// the engine's cell edge instead measures the margin from the far end, which
	// can land a device pixel away from that.
	const qint64 layoutStartDev = sgn > 0 ? 0 : -layoutMainExtentDev;
	const qint64 layoutEndDev = sgn > 0 ? layoutMainExtentDev : 0;
	// The engine's trailing edge in traversal coordinates: only used to keep an
	// over-constrained layout overflowing, as Qt does, instead of pulling its
	// last child back inside the box.
	const auto engineTrailing = [&](const Row& row) {
		return sgn > 0 ? row.mainFarExact : -std::llround(row.mainExact);
	};

	const auto gapDev = [&](int prev, int next) {
		return spacingDev + rows.at(prev).marginTrailDev +
			   rows.at(next).marginLeadDev;
	};

	// Sizes are decided before positions. A gap is measured from the size that
	// actually gets written, so spacing stays equal however a child was rounded,
	// remembered, or filled.
	for (Row& row : rows) {
		qint64 main =
			row.mainOwnDev >= 0
				? row.mainOwnDev
				: (std::max)(std::llround(row.mainSizeExact), qint64(0));
		qint64 cross =
			row.crossOwnDev >= 0
				? row.crossOwnDev
				: (std::max)(horiz ? row.engineHeight : row.engineWidth,
							 qint64(0));

		// Keep the size already written unless the engine asks for a different
		// one, or something else resized the child. An aligned size stays
		// aligned wherever the layout moves, and resizing again costs the child
		// a relayout: an Image reloads its texture, a nested layout rearranges.
		const auto stored = d->childSizes.constFind(row.child);
		if (stored != d->childSizes.constEnd() &&
			stored.value().engineWidth == row.engineWidth &&
			stored.value().engineHeight == row.engineHeight &&
			std::llround(row.child->width() * dpr) == stored.value().width &&
			std::llround(row.child->height() * dpr) == stored.value().height) {
			main = horiz ? stored.value().width : stored.value().height;
			cross = horiz ? stored.value().height : stored.value().width;
		}

		row.size = (std::max)(main, qint64(0));
		row.cross = (std::max)(cross, qint64(0));
	}

	// The first child that fills the main axis absorbs every change in front of
	// it, and every child behind it is measured back from the layout's own end
	// edge. A chain that runs through a fill would otherwise carry a leading
	// child's rounding - and its size change - into children that are supposed
	// to sit still against the far edge.
	int fillIdx = int(rows.size());
	for (int i = 0; i < int(rows.size()); ++i) {
		if (rows.at(i).fillsMain) {
			fillIdx = i;
			break;
		}
	}

	qint64 prevPos = 0;
	qint64 prevSize = 0;
	const int leadEnd = (std::min)(fillIdx, int(rows.size()) - 1);
	for (int i = 0; i <= leadEnd; ++i) {
		Row& row = rows[i];
		row.pos = i == 0 ? layoutStartDev + row.marginLeadDev
						 : prevPos + prevSize + gapDev(i - 1, i);

		prevPos = row.pos;
		prevSize = row.size;
	}

	if (fillIdx + 1 < int(rows.size())) {
		// Measured back from the layout's end edge, by the last child's own
		// rounded trailing margin. Only when the engine says the children need
		// more room than the layout has - an over-constrained layout, which
		// overflows in Qt too - is the engine's own edge used instead.
		const qint64 engineEnd = engineTrailing(rows.back());
		qint64 far = engineEnd > layoutEndDev
						 ? engineEnd
						 : layoutEndDev - rows.back().marginTrailDev;
		for (int i = int(rows.size()) - 1; i > fillIdx; --i) {
			Row& row = rows[i];
			row.pos = far - row.size;
			far = row.pos - gapDev(i - 1, i);
		}

		// Whatever the leading block left over belongs to the filling child.
		Row& fill = rows[fillIdx];
		const qint64 remainder = far - fill.pos;
		if (remainder >= 0) fill.size = remainder;
	} else if (fillIdx < int(rows.size())) {
		// A fill that ends the layout takes the whole space up to its end edge.
		Row& fill = rows[fillIdx];
		const qint64 remainder = layoutEndDev - fill.pos;
		if (remainder >= 0) fill.size = remainder;
	}

	for (Row& row : rows) {
		QQuickItem* child = row.child;

		// Nothing may be written past the layout's own end edge: a child sized
		// from a rounded-up implicit size can reach past it while the layout is
		// still being sized. Corrections are made in steps of two device pixels,
		// so an item that centers its own content (Layout.HCenter /
		// Layout.VCenter, anchors.centerIn) stays on whole device pixels. The
		// layout's edge is the only ceiling - the engine's cell edge is not,
		// because sizes here are rounded up and a child that asks for half a
		// device pixel more than its cell is not shrunk: shrinking it would push
		// the spacing behind it off the normalized value.
		const qint64 mainLimit = (std::max)(layoutEndDev - row.pos, qint64(0));
		qint64 mainSize = row.size;
		if (mainSize > mainLimit) {
			mainSize = mainLimit;
			// Keep the parity of the size the child asks for.
			if (int(mainSize % 2) != int(row.size % 2)) --mainSize;
		}
		mainSize = (std::max)(mainSize, qint64(0));

		const qint64 near = sgn > 0 ? row.pos : -row.pos - mainSize;
		const qint64 width = horiz ? mainSize : row.cross;
		const qint64 height = horiz ? row.cross : mainSize;

		// What is remembered is what the child asked for, not what was written:
		// a size shortened by the ceiling above must be tried again once the
		// layout has room, or the content stays shorter than the layout's own
		// implicit size forever - a Flickable that takes that implicit size as
		// its content height then has bounds that do not match its content.
		d->childSizes[child] = SnapLayoutPrivate::ChildSize{
			row.engineWidth,
			row.engineHeight,
			horiz ? row.size : row.cross,
			horiz ? row.cross : row.size};

		const qint64 nearX = originXDev + (horiz ? near : row.crossExact);
		const qint64 nearY = originYDev + (horiz ? row.crossExact : near);

		const QRectF snapped(
			qreal(nearX) / dpr - origin.x(),
			qreal(nearY) / dpr - origin.y(),
			qreal(width) / dpr,
			qreal(height) / dpr);

		const QSizeF oldSize(child->width(), child->height());
		const QSizeF newSize = snapped.size();

		child->setPosition(snapped.topLeft());
		if (newSize == oldSize) {
			if (QQuickLayout* lay = qobject_cast<QQuickLayout*>(child)) {
				if (lay->invalidatedArrangement()) lay->rearrange(newSize);
			}
		} else {
			child->setSize(newSize);
		}
	}
}

void SnapLayout::rearrange(const QSizeF& size) {
	Q_D(SnapLayout);
	if (!isReady() || !size.isValid()) return;

	const auto refCounter = qScopeGuard([&d] { --(d->recurRearrangeCounter); });
	if (d->recurRearrangeCounter++ == 2) {
		qWarning() << "Qt Quick Layouts: Detected recursive rearrange. "
					  "Aborting after two iterations.";
		return;
	}

	ensureLayoutItemsUpdated(
		QQuickLayout::ApplySizeHints | QQuickLayout::Recursive);

	d->rearranging = true;
	d->engine.setVisualDirection(effectiveLayoutDirection());
	updateAncestorListeners();

	if (const QQuickWindow* win = window()) {
		if (const qreal dpr = win->effectiveDevicePixelRatio(); dpr > 0.0) {
			const QPointF origin = absoluteOrigin(this);
			d->originResidualX = originResidual(origin.x(), dpr);
			d->originResidualY = originResidual(origin.y(), dpr);
		}
	}

	QQuickLayout::rearrange(size);

	for (int i = 0; i < d->engine.itemCount(); ++i)
		static_cast<SnapGridLayoutItem*>(d->engine.itemAt(i))->placed = false;
	d->engine.setGeometries(QRectF(QPointF(0, 0), size), d->styleInfo);
	arrangeChildren();
	d->rearranging = false;

	for (auto childItem : std::as_const(d->invalidateAfterRearrange))
		invalidate(childItem);
	d->invalidateAfterRearrange.clear();
}

} // namespace ZShell::components