TinyMCE: cannot edit as full screen for an XhtmlString property

Description

1. Install the Alloy MVC sample site.
2. Add a property to a StandardPage model:

[Display(GroupName = SystemTabNames.Content, Order = -3000)]
[UIHint("MyModelEditor")]
public virtual IEnumerable<MyModel> Items { get; set; }

3. Add the property to StandardPage view template for editing in OPE mode:

@Html.PropertyFor(x => x.CurrentPage.Items)

4. Build the project.
5. Go to a standard page and try to add items to the property in All Properties / OPE mode.
6. Toggle full screen on the TinyMCE editor.
7. The TinyMCE appears behind the dialog.