Don't prevent default if the user is selecting text in a notebook output.

packages/notebook/src/widget.ts

@@ -1661,7 +1661,10 @@ export class Notebook extends StaticNotebook {
     if (targetArea === 'notebook') {
       this.deselectAll();
     } else if (targetArea === 'prompt' || targetArea === 'cell') {
-      if (button === 0 && shiftKey) {
+      // We don't want to prevent the default selection behavior
+      // if there is currently text selected in an output.
+      const hasSelection = window.getSelection().toString() !== '';
+      if (button === 0 && shiftKey && !hasSelection) {
         // Prevent browser selecting text in prompt or output
         event.preventDefault();
 

tests/test-notebook/src/widget.spec.ts

@@ -1138,6 +1138,21 @@ describe('@jupyter/notebook', () => {
           expect(selected(widget)).to.deep.equal([2, 3]);
         });
 
+        it('should not extend a selection if there is text selected in the output', () => {
+          widget.activeCellIndex = 2;
+
+          // Set a selection in the active cell outputs.
+          const selection = window.getSelection();
+          selection.selectAllChildren(
+            (widget.activeCell as CodeCell).outputArea.node
+          );
+
+          // Shift click below, which should not extend cells selection.
+          simulate(widget.widgets[4].node, 'mousedown', { shiftKey: true });
+          expect(widget.activeCellIndex).to.equal(2);
+          expect(selected(widget)).to.deep.equal([]);
+        });
+
         it('should leave a markdown cell rendered', () => {
           const code = widget.model.contentFactory.createCodeCell({});
           const md = widget.model.contentFactory.createMarkdownCell({});