Add clickable tabs

This introduces a new interface `Clickable`. I'd imagine this would be
implemented for most widgets eventually and would allow for programs run
in the terminal to also have their mouse events forwarded to them.

For the tabs it was relatively simple to check that the position of the
click is within the boxes for the tabs. For other components I'd imagine
that some state representing their currently drawn bounding box would be
useful.

lib/ui/interfaces.go

@@ -40,3 +40,10 @@ type Container interface {
 	// recurse into your grandchildren).
 	Children() []Drawable
 }
+
+// A drawable that can be clicked
+type Clickable interface {
+	Drawable
+
+	MouseEvent(event tcell.Event)
+}

lib/ui/tab.go

@@ -126,6 +126,19 @@ func (tabs *Tabs) removeHistory(index int) {
 	tabs.history = newHist
 }
 
+func (tabs *Tabs) MouseEvent(event tcell.Event) {
+	switch event := event.(type) {
+	case *tcell.EventMouse:
+		if event.Buttons()&tcell.Button1 != 0 {
+			x, y := event.Position()
+			selectedTab, ok := tabs.TabStrip.Clicked(x, y)
+			if ok {
+				tabs.Select(selectedTab)
+			}
+		}
+	}
+}
+
 // TODO: Color repository
 func (strip *TabStrip) Draw(ctx *Context) {
 	x := 0
@@ -151,6 +164,21 @@ func (strip *TabStrip) OnInvalidate(onInvalidate func(d Drawable)) {
 	strip.onInvalidateStrip = onInvalidate
 }
 
+func (strip *TabStrip) Clicked(mouseX int, mouseY int) (int, bool) {
+	x := 0
+	if mouseY == 0 {
+		for i, tab := range strip.Tabs {
+			trunc := runewidth.Truncate(tab.Name, 32, "…")
+			length := len(trunc) + 2
+			if x <= mouseX && mouseX < x+length {
+				return i, true
+			}
+			x += length
+		}
+	}
+	return 0, false
+}
+
 func (content *TabContent) Children() []Drawable {
 	children := make([]Drawable, len(content.Tabs))
 	for i, tab := range content.Tabs {

lib/ui/ui.go

@@ -32,6 +32,7 @@ func Initialize(conf *config.AercConfig,
 
 	screen.Clear()
 	screen.HideCursor()
+	screen.EnableMouse()
 
 	width, height := screen.Size()
 

widgets/aerc.go

@@ -190,6 +190,8 @@ func (aerc *Aerc) Event(event tcell.Event) bool {
 			}
 			return false
 		}
+	case *tcell.EventMouse:
+		aerc.tabs.MouseEvent(event)
 	}
 	return false
 }