Add click-header event for reporting clicks within header (#3768)

Sets $FZF_CLICK_HEADER_LINE and $FZF_CLICK_HEADER_COLUMN env vars with
coordinates of the last click inside and relative to the header and
fires click-header event.

Co-authored-by: Junegunn Choi <junegunn.c@gmail.com>

man/man1/fzf.1

@@ -1278,6 +1278,15 @@ e.g.
      \fBfzf --bind space:jump,jump:accept,jump-cancel:abort\fR
 .RE
 
+\fIclick-header\fR
+.RS
+Triggered when a mouse click occurs within the header. Sets \fBFZF_CLICK_HEADER_LINE\fR and \fBFZF_CLICK_HEADER_COLUMN\fR environment variables.
+
+e.g.
+     \fBprintf "head1\\nhead2" | fzf --header-lines=2 --bind 'click-header:transform-prompt:printf ${FZF_CLICK_HEADER_LINE}x${FZF_CLICK_HEADER_COLUMN}'\fR
+
+.RE
+
 .SS AVAILABLE ACTIONS:
 A key or an event can be bound to one or more of the following actions.
 

src/options.go

@@ -708,6 +708,8 @@ func parseKeyChordsImpl(str string, message string, exit func(string)) map[tui.E
 			add(tui.Jump)
 		case "jump-cancel":
 			add(tui.JumpCancel)
+		case "click-header":
+			add(tui.ClickHeader)
 		case "alt-enter", "alt-return":
 			chords[tui.CtrlAltKey('m')] = key
 		case "alt-space":

src/terminal.go

@@ -298,6 +298,8 @@ type Terminal struct {
 	areaLines          int
 	areaColumns        int
 	forcePreview       bool
+	clickHeaderLine    int
+	clickHeaderColumn  int
 }
 
 type selectedItem struct {
@@ -857,6 +859,8 @@ func (t *Terminal) environ() []string {
 	env = append(env, fmt.Sprintf("FZF_LINES=%d", t.areaLines))
 	env = append(env, fmt.Sprintf("FZF_COLUMNS=%d", t.areaColumns))
 	env = append(env, fmt.Sprintf("FZF_POS=%d", util.Min(t.merger.Length(), t.cy+1)))
+	env = append(env, fmt.Sprintf("FZF_CLICK_HEADER_LINE=%d", t.clickHeaderLine))
+	env = append(env, fmt.Sprintf("FZF_CLICK_HEADER_COLUMN=%d", t.clickHeaderColumn))
 	return env
 }
 
@@ -4001,10 +4005,10 @@ func (t *Terminal) Loop() {
 				}
 
 				if me.Down {
-					mx = util.Constrain(mx-t.promptLen, 0, len(t.input))
-					if my == t.promptLine() && mx >= 0 {
+					mx_cons := util.Constrain(mx-t.promptLen, 0, len(t.input))
+					if my == t.promptLine() && mx_cons >= 0 {
 						// Prompt
-						t.cx = mx + t.xoffset
+						t.cx = mx_cons + t.xoffset
 					} else if my >= min {
 						t.vset(t.offset + my - min)
 						req(reqList)
@@ -4019,6 +4023,34 @@ func (t *Terminal) Loop() {
 							}
 						}
 						return doActions(actionsFor(evt))
+					} else {
+						// Header
+						lineOffset := 0
+						numLines := t.visibleHeaderLines()
+						if !t.headerFirst {
+							// offset for info line
+							if t.noSeparatorLine() {
+								lineOffset = 1
+							} else {
+								lineOffset = 2
+							}
+						} else {
+							// adjust for too-small window
+							numItems := t.areaLines - numLines
+							if !t.noSeparatorLine() {
+								numItems -= 1
+							}
+							if numItems < 0 {
+								numLines += numItems
+							}
+						}
+						my = util.Constrain(my-lineOffset, -1, numLines)
+						mx -= 2 // offset gutter
+						if my >= 0 && my < numLines && mx >= 0 {
+							t.clickHeaderLine = my + 1
+							t.clickHeaderColumn = mx + 1
+							return doActions(actionsFor(tui.ClickHeader))
+						}
 					}
 				}
 			case actReload, actReloadSync:

src/tui/tui.go

@@ -130,6 +130,7 @@ const (
 	Result
 	Jump
 	JumpCancel
+	ClickHeader
 )
 
 func (t EventType) AsEvent() Event {