in src/helpers/edge-helpers.js [269:289]
export function getEdgeHandleRotation(
negate: any = false,
sourceNode: ITargetPosition | null,
targetNode: ITargetPosition
): [string, boolean] {
let rotated = false;
const src = sourceNode;
const trg = targetNode;
let theta = (getTheta(src, trg) * 180) / Math.PI;
if (negate) {
theta = -theta;
}
if (theta > 90 || theta < -90) {
theta = theta + 180;
rotated = true;
}
return [`rotate(${theta})`, rotated];
}