Fix notebook selection issues including from placeholder (#3836)

src/sql/parts/notebook/cellViews/placeholderCell.component.html

@@ -7,7 +7,7 @@
 <div style="overflow: hidden; width: 100%; height: 100%; display: flex; flex-flow: column">
 	<div class="placeholder-cell-component" style="flex: 0 0 auto;">
 		<div class="placeholder-cell-component text">
-			<p>{{clickOn}} <a href="#" (click)="addCell('code')">{{plusCode}}</a> {{or}} <a href="#" (click)="addCell('markdown')">{{plusText}}</a> {{toAddCell}}</p>
+			<p>{{clickOn}} <a href="#" (click)="addCell('code', $event)">{{plusCode}}</a> {{or}} <a href="#" (click)="addCell('markdown', $event)">{{plusText}}</a> {{toAddCell}}</p>
 		</div>
 	</div>
 </div>

src/sql/parts/notebook/cellViews/placeholderCell.component.ts

@@ -68,7 +68,10 @@ export class PlaceholderCellComponent extends CellView implements OnInit, OnChan
 		return localize('toAddCell', 'to add a code or text cell');
 	}
 
-	public addCell(cellType: string): void {
+	public addCell(cellType: string, event?: Event): void {
+		if (event) {
+			event.stopPropagation();
+		}
 		let type: CellType = <CellType>cellType;
 		if (!type) {
 			type = 'code';

src/sql/parts/notebook/notebook.component.ts

@@ -173,14 +173,14 @@ export class NotebookComponent extends AngularDisposable implements OnInit, OnDe
 	public unselectActiveCell() {
 		if (this.model && this.model.activeCell) {
 			this.model.activeCell.active = false;
+			this.model.activeCell = undefined;
 		}
 		this._changeRef.detectChanges();
 	}
 
 	// Add cell based on cell type
 	public addCell(cellType: CellType) {
-		let newCell = this._model.addCell(cellType);
-		this.selectCell(newCell);
+		this._model.addCell(cellType);
 	}
 
 	// Updates Notebook model's trust details