Created attachment 396802 [details]
Screenshot

<rdar://problem/61950751>

Created attachment 397055 [details]
Patch

Comment on attachment 397055 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=397055&action=review

r+ but please consider the comment.

> Source/WebCore/rendering/RenderLayer.cpp:3185
> +    IntRect hBarRect, vBarRect, cornerRect, resizerRect;
> +    getOverflowControlsRects(hBarRect, vBarRect, cornerRect, resizerRect);

I'd much prefer a function that returns a struct over one that requires awkward out parameters.

Created attachment 397090 [details]
Patch

Comment on attachment 397055 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=397055&action=review

> Source/WebCore/rendering/RenderLayer.cpp:3364
> +    auto cornerRect = [](const IntRect& boundsRect, IntSize cornerSize, bool placeVerticalScrollbarOnTheLeft) {
> +        if (placeVerticalScrollbarOnTheLeft) {
> +            auto bottomLeftCorner = boundsRect.minXMaxYCorner();
> +            return IntRect { { bottomLeftCorner.x(), bottomLeftCorner.y() - cornerSize.height(), }, cornerSize };
> +        }
> +        return IntRect { boundsRect.maxXMaxYCorner() - cornerSize, cornerSize };
> +    };
> +
> +    if (scrollbarsAvoidCorner)
> +        scrollCornerRect = cornerRect(overflowControlsPositioningRect, cornerSize, placeVerticalScrollbarOnTheLeft);
> +    else
> +        scrollCornerRect = { };

You could include 'if (!scrollbarsAvoidCorner)' branch into the lambda as well. 
Not sure this single use lambda really needs arguments at all, it could just capture the variables: [&] {

Other branches in this function would probably benefit from similar use of lambdas.

https://trac.webkit.org/changeset/260445/webkit