Just changing outline color sets "recompute_overflow" flag on StyleDifference object.

LayoutObject::SetStyle uses this flag to invalidate layout:

LayoutObject::SetStyle 
 if (diff.NeedsRecomputeOverflow() && !NeedsLayout()) {
   SetNeedsLayoutAndPrefWidthsRecalc(LayoutInvalidationReason::kStyleChange);

The code that determines whether overflow recomputation is needed is at:

ComputedStyle::UpdatePropertySpecificDifferences
   ComputedStyleBase::UpdatePropertySpecificDifferencesNeedsRecomputeOverflow
     if (!a.OutlineVisuallyEqual(b))
       return true;

Outlines can be visually different (color change), but not trigger overflow computation.

I think we need to separate OutlineVisuallyEqual into OutlineLayoutEqual, OutlinePaintEqual.

One should trigger overflow computation, and the other should not.

Overflow computation in LayoutNG triggers full layout for now.