fix(physarum): paint only while the pointer is pressed

Ignore pointer moves after release.

src/app/pages/algorithms/physarum/physarum.component.ts

@@ -70,7 +70,8 @@ export class PhysarumComponent implements OnDestroy {
   private canvas: HTMLCanvasElement | null = null;
   private pointerAttached = false;
   private agentCountInitialized = false;
-  private readonly pointerLeaveHandler = () => { this.food.active = false; };
+  private pressed = false;
+  private readonly pointerLeaveHandler = () => { this.pressed = false; this.food.active = false; };
   private readonly fpsCounter = new FpsCounter();
 
   readonly minAgents = MIN_AGENTS;
@@ -231,17 +232,16 @@ export class PhysarumComponent implements OnDestroy {
 
     scene.onPointerObservable.add((info) => {
       if (info.type === PointerEventTypes.POINTERUP) {
+        this.pressed = false;
         this.food.active = false;
         return;
       }
-      if (info.type !== PointerEventTypes.POINTERDOWN && info.type !== PointerEventTypes.POINTERMOVE) {
+      if (info.type === PointerEventTypes.POINTERDOWN) {
+        this.pressed = true;
+      } else if (info.type !== PointerEventTypes.POINTERMOVE || !this.pressed) {
         return;
       }
       const event = info.event as PointerEvent;
-      if (!(event.buttons & 1) && event.pointerType !== 'touch') {
-        this.food.active = false;
-        return;
-      }
       const target = event.target as HTMLElement;
       const cell = pointerToCell(event.offsetX, event.offsetY, target.clientWidth, target.clientHeight, this.mapSize(), this.mapSize());
       this.food.x = cell.x;