Decision must not depend on threshold chain resolution
(computePreviewSize consults noSeparatorLine); separator is hidden only
when every spec in the chain displays a 'next' preview window with a
border line facing the input section. Otherwise err on the side of
showing it.
- Without active previewer, preview position is resolved from first spec
alone, so 'next' in a threshold alternative does not create input
window and cannot hide the separator
- Empty preview command is undecidable; preview(...) action can display
ad-hoc window at 'next' position at any time
- change-preview-window relayouts or repaints when separator visibility
changes even if the active spec is unchanged, without dropping preview
window forced by preview(...) action
Junegunn Choi
14755d683a