Most of the decorative items associated with a file offset are
placed before the item in the display list, and given a span of
zero. This yields the correct behavior in a binary search: an
exact match finds the decorative item (e.g. a blank line), while a
match partway into the instruction or multi-byte data item causes
the binary search to move on to the next line, where it's resolved.
The problem is that we were adding a blank line *after* instructions
in the no-continue case. If the binary search found the blank line
before it found the instruction, it would guess "too high" rather
than "too low", and miss the actual instruction line.
We now set a flag and add the blank line as part of the following
item. We do a little dance at the start to ensure that the blank
line doesn't disappear during a partial update.
It's possible to have format descriptors on instructions that are
left over from when the bytes were treated as data. Single-byte
formats were being allowed on single-byte instructions, which
confused things later when the code tried to apply the format to
an instruction with no operand.
