Preserve node angles when shaping bezier segments

When shaping a bezier segment by click/dragging the Node tool across the segment itself, it would be nice to have a modifier key available (Ctrl?) to preserve the angles of its adjacent node handles and adjust only their lengths.

This could be useful for building typography -- some font formats require that beziers do not exceed the bounding box established by their endpoints, with the side effect that certain nodes are oriented strictly horizontal and/or vertical.