Add 'focus' event

Can we find a better name? I have considered the followings.

* 'point', because "the pointer" points to the current item.
* 'shift', 'switch', 'move', etc. These are not technically correct
  because the current item can change without cursor movement (--tac,
  reload, search update)
* 'change' is already taken. 'change-current' feels a bit wordy and
  sounds wrong, 'current-changed' is wordy and doesn't go well with the
  other event names
* 'target', not straightforward

Close #3053

src/terminal.go

@@ -1265,7 +1265,7 @@ func (t *Terminal) resizeWindows(forcePreview bool) {
 }
 
 func (t *Terminal) printLabel(window tui.Window, render labelPrinter, opts labelOpts, length int, borderShape tui.BorderShape, redrawBorder bool) {
-	if window == nil || render == nil {
+	if window == nil {
 		return
 	}
 
@@ -1274,6 +1274,9 @@ func (t *Terminal) printLabel(window tui.Window, render labelPrinter, opts label
 		if redrawBorder {
 			window.DrawHBorder()
 		}
+		if render == nil {
+			return
+		}
 		var col int
 		if opts.column == 0 {
 			col = util.Max(0, (window.Width()-length)/2)
@@ -2616,6 +2619,11 @@ func (t *Terminal) Loop() {
 		}
 	}
 
+	var onFocus []*action
+	if actions, prs := t.keymap[tui.Focus.AsEvent()]; prs {
+		onFocus = actions
+	}
+
 	go func() {
 		var focusedIndex int32 = minItem.Index()
 		var version int64 = -1
@@ -2651,7 +2659,11 @@ func (t *Terminal) Loop() {
 						if currentItem != nil {
 							currentIndex = currentItem.Index()
 						}
-						if focusedIndex != currentIndex || version != t.version {
+						focusChanged := focusedIndex != currentIndex
+						if onFocus != nil && focusChanged {
+							t.serverChan <- onFocus
+						}
+						if focusChanged || version != t.version {
 							version = t.version
 							focusedIndex = currentIndex
 							refreshPreview(t.previewOpts.command)