Fix non-working mouse click on top-left and top-right cells

Scroll the tab bar only if there actually is the tab bar, otherwise
propagate the mouse click event to the bufpane.

internal/action/tab.go

@@ -107,14 +107,15 @@ func (t *TabList) HandleEvent(event tcell.Event) {
 		mx, my := e.Position()
 		switch e.Buttons() {
 		case tcell.Button1:
-			if my == t.Y && mx == 0 {
-				t.Scroll(-4)
-				return
-			} else if my == t.Y && mx == t.Width-1 {
-				t.Scroll(4)
-				return
-			}
 			if len(t.List) > 1 {
+				if my == t.Y && mx == 0 {
+					t.Scroll(-4)
+					return
+				} else if my == t.Y && mx == t.Width-1 {
+					t.Scroll(4)
+					return
+				}
+
 				ind := t.LocFromVisual(buffer.Loc{mx, my})
 				if ind != -1 {
 					t.SetActive(ind)