Once a DynamicText is added it can break the editor

[stevejpurves]

This is difficult to reproduce reliably but when adding dynamic elements to text I have encountered:

• A dynamic text element is suddenly being replicated twice in the text. This happened on copy-paste of a different adjacent piece of text and (2) when typing a character ahead of a dynamic text.
• A dynamic text element is inserted at a location when triggering the / menu, after the element appears the editor is not 100% functional. the element inserted was a copy of the last one that I inserted manually. on page refresh the additional element is no longer present.

The following image does not show the issue but shows the piece of text I was working on when I hit these problems (note there are several more dynamic text elements higher up the page)

[rowanc1]

I think this is chrome+content-editable+webcomponents.

Solution is likely to move away from webcomponents for things that go in the editor.