fixes for non-dual-node rbf feature

frontend/src/app/components/rbf-timeline/rbf-timeline-tooltip.component.html

@@ -1,6 +1,6 @@
 <div
   #tooltip
-  *ngIf="rbfInfo"
+  *ngIf="rbfInfo && tooltipPosition !== null"
   class="rbf-tooltip"
   [style.left]="tooltipPosition.x + 'px'"
   [style.top]="tooltipPosition.y + 'px'"

frontend/src/app/components/rbf-timeline/rbf-timeline-tooltip.component.ts

@@ -10,7 +10,7 @@ export class RbfTimelineTooltipComponent implements OnChanges {
   @Input() rbfInfo: RbfInfo | void;
   @Input() cursorPosition: { x: number, y: number };
 
-  tooltipPosition = { x: 0, y: 0 };
+  tooltipPosition = null;
 
   @ViewChild('tooltip') tooltipElement: ElementRef<HTMLCanvasElement>;
 

frontend/src/app/components/rbf-timeline/rbf-timeline.component.ts

@@ -23,7 +23,7 @@ export class RbfTimelineComponent implements OnInit, OnChanges {
   rows: TimelineCell[][] = [];
 
   hoverInfo: RbfInfo | void = null;
-  tooltipPosition = { x: 0, y: 0 };
+  tooltipPosition = null;
 
   dir: 'rtl' | 'ltr' = 'ltr';