A sheet takes the place of the window. No element pairs, so the numbers start at 1,
and the diff would cost 3 lines against a tree of 2: the full tree is printed. The
next read is a diff against it. A read with diffing off counts from 0 again.
== read
a window Main
.b button OK
== want
Window: "Main", App: App.
0 window Main
	1 button OK

== read title=Unknown
x sheet
.y button Save
== want
Window: "Unknown", App: App.
1 sheet
	2 button Save

== read title=Unknown
x sheet
.y button Saved
== want
<HEADER>
~	2 button Saved
== read full title=Unknown
x sheet
.y button Saved
== want
Window: "Unknown", App: App.
0 sheet
	1 button Saved

== end
