ContextualToolbar should have limiter set to editable element

[oleq]

Expected:

Actual:

The following should do the trick:

diff --git a/src/toolbar/contextual/contextualtoolbar.js b/src/toolbar/contextual/contextualtoolbar.js
index 5e7ad47..389d5f4 100644
--- a/src/toolbar/contextual/contextualtoolbar.js
+++ b/src/toolbar/contextual/contextualtoolbar.js
@@ -231,6 +231,7 @@ export default class ContextualToolbar extends Plugin {
 				// Select the proper range rect depending on the direction of the selection.
 				return isBackward ? rangeRects.item( 0 ) : rangeRects.item( rangeRects.length - 1 );
 			},
+			limiter: this.editor.ui.view.editable.element,
 			positions: isBackward ?
 				[ defaultPositions.northWestArrowSouth, defaultPositions.southWestArrowNorth ] :
 				[ defaultPositions.southEastArrowNorth, defaultPositions.northEastArrowSouth ]

Closely related to https://github.com/ckeditor/ckeditor5-ui/issues/181 and https://github.com/ckeditor/ckeditor5-ui/issues/223.