static void updateLineConstrains(const RenderBlockFlow& flow, LineState& line, bool isFirstLine)
{
    bool shouldApplyTextIndent = !flow.isAnonymous() || flow.parent()->firstChild() == &flow;
    LayoutUnit height = flow.logicalHeight();
    LayoutUnit logicalHeight = flow.minLineHeightForReplacedRenderer(false, 0);
    float logicalRightOffset = flow.logicalRightOffsetForLine(height, false, logicalHeight);
    line.setLogicalLeftOffset(flow.logicalLeftOffsetForLine(height, false, logicalHeight) + (shouldApplyTextIndent && isFirstLine ? flow.textIndentOffset() : LayoutUnit(0)));
    line.setAvailableWidth(std::max<float>(0, logicalRightOffset - line.logicalLeftOffset()));
}
static AvoidanceReasonFlags canUseForWithReason(const RenderBlockFlow& flow, FallThrough fallthrough)
{
#ifndef NDEBUG
    static std::once_flag onceFlag;
    std::call_once(onceFlag, [] {
        registerNotifyCallback("com.apple.WebKit.showSimpleLineLayoutCoverage", printSimpleLineLayoutCoverage);
        registerNotifyCallback("com.apple.WebKit.showSimpleLineLayoutReasons", printSimpleLineLayoutBlockList);
    });
#endif
    AvoidanceReasonFlags reasons = NoReason;
    if (!flow.frame().settings().simpleLineLayoutEnabled())
        SET_REASON_AND_RETURN_IF_NEEDED(reasons, FeatureIsDisabled, fallthrough);
    if (!flow.parent())
        SET_REASON_AND_RETURN_IF_NEEDED(reasons, FlowHasNoParent, fallthrough);
    if (!flow.firstChild())
        SET_REASON_AND_RETURN_IF_NEEDED(reasons, FlowHasNoChild, fallthrough);
    if (flow.flowThreadState() != RenderObject::NotInsideFlowThread)
        SET_REASON_AND_RETURN_IF_NEEDED(reasons, FlowIsInsideRegion, fallthrough);
    if (!flow.isHorizontalWritingMode())
        SET_REASON_AND_RETURN_IF_NEEDED(reasons, FlowHasHorizonalWritingMode, fallthrough);
    if (flow.hasOutline())
        SET_REASON_AND_RETURN_IF_NEEDED(reasons, FlowHasOutline, fallthrough);
    if (flow.isRubyText() || flow.isRubyBase())
        SET_REASON_AND_RETURN_IF_NEEDED(reasons, FlowIsRuby, fallthrough);
    // Printing does pagination without a flow thread.
    if (flow.document().paginated())
        SET_REASON_AND_RETURN_IF_NEEDED(reasons, FlowIsPaginated, fallthrough);
    if (flow.isAnonymous() && flow.firstLineBlock())
        SET_REASON_AND_RETURN_IF_NEEDED(reasons, FlowHasPseudoFirstLine, fallthrough);
    if (flow.isAnonymousBlock() && flow.parent()->style().textOverflow())
        SET_REASON_AND_RETURN_IF_NEEDED(reasons, FlowHasTextOverflow, fallthrough);
    if (flow.parent()->isDeprecatedFlexibleBox())
        SET_REASON_AND_RETURN_IF_NEEDED(reasons, FlowIsDepricatedFlexBox, fallthrough);
    // FIXME: Placeholders do something strange.
    if (is<RenderTextControl>(*flow.parent()) && downcast<RenderTextControl>(*flow.parent()).textFormControlElement().placeholderElement())
        SET_REASON_AND_RETURN_IF_NEEDED(reasons, FlowParentIsPlaceholderElement, fallthrough);
    // FIXME: Implementation of wrap=hard looks into lineboxes.
    if (flow.parent()->isTextArea() && flow.parent()->element()->fastHasAttribute(HTMLNames::wrapAttr))
        SET_REASON_AND_RETURN_IF_NEEDED(reasons, FlowParentIsTextAreaWithWrapping, fallthrough);
    // This currently covers <blockflow>#text</blockflow>, <blockflow>#text<br></blockflow> and mutiple (sibling) RenderText cases.
    // The <blockflow><inline>#text</inline></blockflow> case is also popular and should be relatively easy to cover.
    for (const auto* child = flow.firstChild(); child;) {
        if (is<RenderText>(*child)) {
            child = child->nextSibling();
            continue;
        }
        if (is<RenderLineBreak>(child) && !downcast<RenderLineBreak>(*child).isWBR() && child->style().clear() == CNONE) {
            child = child->nextSibling();
            continue;
        }
        SET_REASON_AND_RETURN_IF_NEEDED(reasons, FlowHasNonSupportedChild, fallthrough);
        break;
    }
    auto styleReasons = canUseForStyle(flow.style(), fallthrough);
    if (styleReasons != NoReason)
        SET_REASON_AND_RETURN_IF_NEEDED(reasons, styleReasons, fallthrough);
    // We can't use the code path if any lines would need to be shifted below floats. This is because we don't keep per-line y coordinates.
    if (flow.containsFloats()) {
        float minimumWidthNeeded = std::numeric_limits<float>::max();
        for (const auto& textRenderer : childrenOfType<RenderText>(flow)) {
            minimumWidthNeeded = std::min(minimumWidthNeeded, textRenderer.minLogicalWidth());

            for (auto& floatingObject : *flow.floatingObjectSet()) {
                ASSERT(floatingObject);
#if ENABLE(CSS_SHAPES)
                // if a float has a shape, we cannot tell if content will need to be shifted until after we lay it out,
                // since the amount of space is not uniform for the height of the float.
                if (floatingObject->renderer().shapeOutsideInfo())
                    SET_REASON_AND_RETURN_IF_NEEDED(reasons, FlowHasUnsupportedFloat, fallthrough);
#endif
                float availableWidth = flow.availableLogicalWidthForLine(floatingObject->y(), false);
                if (availableWidth < minimumWidthNeeded)
                    SET_REASON_AND_RETURN_IF_NEEDED(reasons, FlowHasUnsupportedFloat, fallthrough);
            }
        }
    }
    auto fontAndTextReasons = canUseForFontAndText(flow, fallthrough);
    if (fontAndTextReasons != NoReason)
        SET_REASON_AND_RETURN_IF_NEEDED(reasons, fontAndTextReasons, fallthrough);
    return reasons;
}