fix: prevent Enter/Space from propagating from Grid (#2570)

This can cause unexpected interactions with the keyboard navigation plugin and
potentially other listeners.

We also plan to fix keyboard navigation to ignore events during ephemeral
focus.

See RaspberryPiFoundation/blockly-keyboard-experimentation#618

Author: microbit-matt-hillsdon

plugins/field-grid-dropdown/src/grid.ts

@@ -189,6 +189,12 @@ export class Grid {
       case 'End':
         this.moveFocus(this.items.length - 1, false);
         break;
+      case 'Enter':
+      case 'Space':
+        // Handled via GridItem click handler, so we want its default but it
+        // must not propagate.
+        e.stopPropagation();
+        return;
       default:
         // Not a key the grid is interested in.
         return;