Add the environment variable FZF_PREVIEW_LABEL

The environment variable get the value of the preview label, even if it
has been updated with an action. It can be useful to track the label of
the preview and be able to switch between previews using only one
binding.

man/man1/fzf.1

@@ -975,6 +975,8 @@ fzf exports the following environment variables to its child processes.
 .br
 .BR FZF_PROMPT "          Prompt string"
 .br
+.BR FZF_PREVIEW_LABEL "   Preview label string"
+.br
 .BR FZF_ACTION "          The name of the last action performed"
 .br
 .BR FZF_PORT "            Port number when --listen option is used"

src/terminal.go

@@ -850,6 +850,7 @@ func (t *Terminal) environ() []string {
 	env = append(env, "FZF_QUERY="+string(t.input))
 	env = append(env, "FZF_ACTION="+t.lastAction.Name())
 	env = append(env, "FZF_PROMPT="+string(t.promptString))
+	env = append(env, "FZF_PREVIEW_LABEL="+t.previewLabelOpts.label)
 	env = append(env, fmt.Sprintf("FZF_TOTAL_COUNT=%d", t.count))
 	env = append(env, fmt.Sprintf("FZF_MATCH_COUNT=%d", t.merger.Length()))
 	env = append(env, fmt.Sprintf("FZF_SELECT_COUNT=%d", len(t.selected)))
@@ -3476,6 +3477,7 @@ func (t *Terminal) Loop() {
 					req(reqRedrawBorderLabel)
 				}
 			case actChangePreviewLabel:
+				t.previewLabelOpts.label = a.a
 				if t.pborder != nil {
 					t.previewLabel, t.previewLabelLen = t.ansiLabelPrinter(a.a, &tui.ColPreviewLabel, false)
 					req(reqRedrawPreviewLabel)
@@ -3491,8 +3493,9 @@ func (t *Terminal) Loop() {
 					req(reqRedrawBorderLabel)
 				}
 			case actTransformPreviewLabel:
+				label := t.executeCommand(a.a, false, true, true, true)
+				t.previewLabelOpts.label = label
 				if t.pborder != nil {
-					label := t.executeCommand(a.a, false, true, true, true)
 					t.previewLabel, t.previewLabelLen = t.ansiLabelPrinter(label, &tui.ColPreviewLabel, false)
 					req(reqRedrawPreviewLabel)
 				}