An overflow:hidden fieldset:

The fieldset background's blue top shows where the top-edge of the padding-box is (just inside the border).

The yellow DIV shows whether and where the legend is being clipped by overflow:hidden on the fieldset.

The cyan DIV shows whether and where the normal fieldset children are being clipped by overflow:hidden on the fieldset.

An overflow:scroll fieldset:

The fieldset background's blue top shows where the top-edge of the padding-box is (just inside the border).

The yellow DIV shows whether and where the legend is being clipped by overflow:hidden on the fieldset.

The cyan DIV shows whether and where the normal fieldset children are being clipped by overflow:hidden on the fieldset.

An abs-pos container fieldset:

The yellow DIV shows the abs-pos containing block for children of the legend.

The cyan DIV shows the abs-pos containing block for normal children.

An overflow:scroll abs-pos container fieldset:

The yellow DIV shows the abs-pos containing block for children of the legend.

The cyan DIV shows the abs-pos containing block for normal children.

Does the fieldset border-top affect getBoundingClientRect()?