The layout editor now handles <include> tags, as well as <merge> tags, properly*.
If your code includes other views, those other views are rendered in place into the current page. You can click to select these views, but it will select the entire include block instead, and if you for example hit the Delete key, you will delete the <include> tag in the source view rather than affect the included views. You can double click on these included views to warp into the included layout instead.
The layout editor also detects cycles in your <include> chains and lists these in the Problems view.
*: There is one remaining bug: for design-time rendering of included views, you have to specify width and height attributes on the include tag itself, not just on the root element of the included view.