task(B3): offset respects click side (fixes BUG-3)

frontend/src/interaction/index.ts

@@ -761,7 +761,23 @@ export class InteractionEngine {
       if (this.modifySelected.length === 1) {
         const first = this.state.points[0];
         const dist = distance(first, pt);
-        const offsetted = offsetElement(this.modifySelected[0], dist);
+        const base = this.modifySelected[0];
+        // Seitigkeit bestimmen: Kreuzprodukt der Linienrichtung mit Klickvektor.
+        // cross > 0 → Klick links der Laufrichtung (p1→p2) → side=+1, sonst -1.
+        let side: 1 | -1 = 1;
+        if (base.type === 'line' && base.properties.x1 !== undefined && base.properties.y1 !== undefined) {
+          const p1x = base.properties.x1 as number;
+          const p1y = base.properties.y1 as number;
+          const p2x = base.properties.x2 as number;
+          const p2y = base.properties.y2 as number;
+          const cross = (p2x - p1x) * (pt.y - p1y) - (p2y - p1y) * (pt.x - p1x);
+          side = cross >= 0 ? 1 : -1;
+        } else if ((base.type === 'circle' || base.type === 'arc') && base.properties.radius !== undefined) {
+          // Kreis/Bogen: Klick außerhalb → vergrößern (+1), innerhalb → verkleinern (-1)
+          const clickDist = Math.hypot(pt.x - base.x, pt.y - base.y);
+          side = clickDist >= base.properties.radius ? 1 : -1;
+        }
+        const offsetted = offsetElement(base, dist, side);
         // Create as new element (copy)
         this.onElementCreated?.({ ...offsetted, id: this.generateId() });
         this.resetModify();

frontend/src/tools/modification/geometry.ts

@@ -163,7 +163,14 @@ export function mirrorElement(el: CADElement, x1: number, y1: number, x2: number
  * Offset element by a distance (creates a parallel copy).
  * For lines: offset perpendicular. For circles/arcs: adjust radius.
  */
-export function offsetElement(el: CADElement, distance: number): CADElement {
+/**
+ * Versetzt ein Element senkrecht zur seiner Richtung.
+ * @param distance positiver Betrag des Versatzes
+ * @param side 1 = links der Laufrichtung (Standard), -1 = rechts —
+ *              bei circle/arc: 1 = Radius vergrößern, -1 = verkleinern
+ */
+export function offsetElement(el: CADElement, distance: number, side?: 1 | -1): CADElement {
+  const signed = distance * (side ?? 1);
   const props = { ...el.properties };
 
   if (el.type === 'line' && props.x1 !== undefined && props.y1 !== undefined && props.x2 !== undefined && props.y2 !== undefined) {
@@ -174,8 +181,8 @@ export function offsetElement(el: CADElement, distance: number): CADElement {
     // Perpendicular unit vector
     const nx = -dy / len;
     const ny = dx / len;
-    const ox = nx * distance;
-    const oy = ny * distance;
+    const ox = nx * signed;
+    const oy = ny * signed;
     props.x1 += ox;
     props.y1 += oy;
     props.x2 += ox;
@@ -184,7 +191,7 @@ export function offsetElement(el: CADElement, distance: number): CADElement {
   }
 
   if ((el.type === 'circle' || el.type === 'arc') && props.radius !== undefined) {
-    props.radius = Math.abs(props.radius + distance);
+    props.radius = Math.abs(props.radius + signed);
     const d = props.radius * 2;
     return { ...el, width: d, height: d, properties: props };
   }
@@ -201,7 +208,7 @@ export function offsetElement(el: CADElement, distance: number): CADElement {
       if (len === 0) return p;
       const nx = -dy / len;
       const ny = dx / len;
-      return { x: p.x + nx * distance, y: p.y + ny * distance };
+      return { x: p.x + nx * signed, y: p.y + ny * signed };
     });
     return { ...el, properties: props };
   }

frontend/tests/bugs.regress.test.ts

@@ -5,7 +5,7 @@
  */
 import { describe, it, expect } from 'vitest';
 import { SeatingService } from '../src/services/seatingService';
-import { trimElement } from '../src/tools/modification/geometry';
+import { trimElement, offsetElement } from '../src/tools/modification/geometry';
 import type { CADElement } from '../src/types/cad.types';
 
 // ── Helfer (Muster aus tests/geometry.test.ts) ─────────────
@@ -69,3 +69,34 @@ describe('BUG-2: trim line at circle', () => {
     expect(result).not.toBeNull();
   });
 });
+
+// ── BUG-3: Offset ignorierte die Klickseite ─────────────────
+describe('BUG-3: offset respects click side', () => {
+  it('side=-1 versetzt Linie entgegen der Normalenrichtung', () => {
+    // Linie (100,100)→(300,100): Laufrichtung +x.
+    // Normale (-dy,dx)=(0,1) zeigt in Canvas-Koordinaten nach UNTEN (+y).
+    const line = makeLine('l1', 100, 100, 300, 100);
+    const result = offsetElement(line, 20, -1);
+    expect(result.properties.y1).toBeCloseTo(80); // nach OBEN versetzt
+    expect(result.properties.y2).toBeCloseTo(80);
+  });
+
+  it('side=+1 versetzt Linie in Normalenrichtung (Default wie vorher)', () => {
+    const line = makeLine('l1', 100, 100, 300, 100);
+    const result = offsetElement(line, 20, 1);
+    expect(result.properties.y1).toBeCloseTo(120); // nach UNTEN versetzt
+    expect(result.properties.y2).toBeCloseTo(120);
+  });
+
+  it('ohne side bleibt Verhalten identisch zur alten API (Rueckwaertskompatibilitaet)', () => {
+    const line = makeLine('l1', 100, 100, 300, 100);
+    const result = offsetElement(line, 20);
+    expect(result.properties.y1).toBeCloseTo(120);
+  });
+
+  it('Kreis: side=-1 verkleinert Radius, side=+1 vergroessert', () => {
+    const circle = makeCircle('c1', 0, 0, 50);
+    expect(offsetElement(circle, 10, -1).properties.radius).toBeCloseTo(40);
+    expect(offsetElement(circle, 10, 1).properties.radius).toBeCloseTo(60);
+  });
+});