fix #709

if we do not update on mouseleave user could start a drag inside editor
and end it outside, which will not trigger mouseup since it is not on
the editor, thus we need to update on mosueleave. Earlier this was
removed because of issues with Bubble but those issues are also and
better fixed with preventDefault on mousedown on a button, instead of
click

core/selection.js

@@ -24,7 +24,7 @@ class Selection {
     this.cursor = Parchment.create('cursor', this);
     // savedRange is last non-null range
     this.lastRange = this.savedRange = new Range(0, 0);
-    ['keyup', 'mouseup', 'touchend', 'touchleave', 'focus', 'blur'].forEach((eventName) => {
+    ['keyup', 'mouseup', 'mouseleave', 'touchend', 'touchleave', 'focus', 'blur'].forEach((eventName) => {
       this.root.addEventListener(eventName, () => {
         // When range used to be a selection and user click within the selection,
         // the range now being a cursor has not updated yet without setTimeout