Block Editor: Fix single block selection when holding 'Shift' key (#70978)

Co-authored-by: Mamaduka <[email protected]>
Co-authored-by: tyxla <[email protected]>

Author: Mamaduka

packages/block-editor/src/components/writing-flow/use-click-selection.js

@@ -27,7 +27,9 @@ export default function useClickSelection() {
 				const clickedClientId = getBlockClientId( event.target );
 
 				if ( event.shiftKey ) {
-					if ( startClientId !== clickedClientId ) {
+					// When selecting a single block in a document by holding the shift key,
+					// don't mark this action as multiselection.
+					if ( startClientId && startClientId !== clickedClientId ) {
 						node.contentEditable = true;
 						// Firefox doesn't automatically move focus.
 						node.focus();

test/e2e/specs/editor/various/multi-block-selection.spec.js

@@ -291,20 +291,15 @@ test.describe( 'Multi-block selection (@firefox, @webkit)', () => {
 		editor,
 		pageUtils,
 	} ) => {
-		// To do: run with iframe.
-		await editor.switchToLegacyCanvas();
-
 		await editor.insertBlock( {
 			name: 'core/paragraph',
 			attributes: { content: 'test' },
 		} );
 
 		await editor.saveDraft();
 		await page.reload();
-		// To do: run with iframe.
-		await editor.switchToLegacyCanvas();
 
-		await page
+		await editor.canvas
 			.getByRole( 'document', { name: 'Block: Paragraph' } )
 			.click( { modifiers: [ 'Shift' ] } );
 		await pageUtils.pressKeys( 'primary+a' );