fire onSelectionChange on empty selection

Fire the onSelectionChange event when a selection is cleared by
single-clicking somewhere inside the terminal area.

fixes #3193.

This also slightly changes the behaviour when re-selecting the same area
in the terminal. Before, this fired onSelectionChange, but it does not
anymore, even when changing the direction of the selection.

src/browser/services/SelectionService.ts

@@ -105,6 +105,9 @@ export class SelectionService extends Disposable implements ISelectionService {
   private _workCell: CellData = new CellData();
 
   private _mouseDownTimeStamp: number = 0;
+  private _oldHasSelection: boolean = false;
+  private _oldSelectionStart: [number, number] | undefined = undefined;
+  private _oldSelectionEnd: [number, number] | undefined = undefined;
 
   private _onLinuxMouseSelection = this.register(new EventEmitter<string>());
   public get onLinuxMouseSelection(): IEvent<string> { return this._onLinuxMouseSelection.event; }
@@ -681,7 +684,26 @@ export class SelectionService extends Disposable implements ISelectionService {
           this._coreService.triggerDataEvent(sequence, true);
         }
       }
-    } else if (this.hasSelection) {
+    } else {
+      this._fireIfSelectionChanged();
+    }
+  }
+
+  private _fireIfSelectionChanged(): void {
+    const hasSelection = this.hasSelection;
+    if (!hasSelection && !this._oldHasSelection) {
+      return;
+    }
+
+    const start = this._model.finalSelectionStart;
+    const end = this._model.finalSelectionEnd;
+
+    if (start?.[0] !== this._oldSelectionStart?.[0] || start?.[1] !== this._oldSelectionStart?.[1] ||
+      end?.[0] !== this._oldSelectionEnd?.[0] || end?.[1] !== this._oldSelectionEnd?.[1]) {
+
+      this._oldSelectionStart = start;
+      this._oldSelectionEnd = end;
+      this._oldHasSelection = hasSelection;
       this._onSelectionChange.fire();
     }
   }