There seems to be more than one issues with this
A simpler example is the horizontal rule which also has contenteditable=false
here are some problems I noticed:
go to http://prosemirror.net/examples/basic/ select all and delete
insert horizontal rule now press home and/or end problem - the cursor blinks but it’s not clear if it’s before or after the rule pressing home or end keeps the cursor in the same position
now press x problem - the rule is deleted even though it was not selected
again select and delete all type x insert horizontal rule type ctrl+home (cursor at doc beginning before the x) press arrow fw three times problem - the cursor moves to after the x (expected it after the ruler, this is what I reported above with my link)
type ctrl+home (cursor at doc beginning before the x) type ctrl+end problem - cursor blinks after the rule but them moves to after the x