Do not show preview window by default if --preview is empty

Close #2516

CHANGELOG.md

@@ -1,6 +1,22 @@
 CHANGELOG
 =========
 
+0.27.3
+------
+- Preview window is `hidden` by default when there are `preview` bindings but
+  `--preview` command is not given
+- Vim plugin
+    - `sinklist` is added as a synonym to `sink*` so that it's easier to add
+      a function to a spec dictionary
+      ```vim
+      let spec = { 'source': 'ls', 'options': ['--multi', '--preview', 'cat {}'] }
+      function spec.sinklist(matches)
+        echom string(a:matches)
+      endfunction
+
+      call fzf#run(fzf#wrap(spec))
+      ```
+
 0.27.2
 ------
 - 16 base ANSI colors can be specified by their names

src/terminal.go

@@ -425,6 +425,7 @@ func NewTerminal(opts *Options, eventBox *util.EventBox) *Terminal {
 		delay = initialDelay
 	}
 	var previewBox *util.EventBox
+	showPreviewWindow := len(opts.Preview.command) > 0 && !opts.Preview.hidden
 	if len(opts.Preview.command) > 0 || hasPreviewAction(opts) {
 		previewBox = util.NewEventBox()
 	}
@@ -521,7 +522,7 @@ func NewTerminal(opts *Options, eventBox *util.EventBox) *Terminal {
 		selected:    make(map[int32]selectedItem),
 		reqBox:      util.NewEventBox(),
 		previewOpts: opts.Preview,
-		previewer:   previewer{0, []string{}, 0, previewBox != nil && !opts.Preview.hidden, false, true, false, ""},
+		previewer:   previewer{0, []string{}, 0, showPreviewWindow, false, true, false, ""},
 		previewed:   previewed{0, 0, 0, false},
 		previewBox:  previewBox,
 		eventBox:    eventBox,