ExperimentalThe range or element to measure.
The target's bounding rect in viewport coordinates, or undefined when it paints
nothing: a range whose text the editor replaced (it collapsed to an element boundary that has
no box), or an element with no layout because it or an ancestor is display: none, as inside
an inactive rc-dock tab pane.
This export is unstable and may change shape or disappear without notice
The current viewport rect of a text range or an element, for a LivePopoverAnchorSource's
measure.A target counts as unmeasurable only when it paints no box at all (
getClientRects()is empty), never merely because its box is small: a zero-width or zero-height box — a collapsed caret, an empty element — is still a real, positioned point.