compose: fix mouse focus of header editors

The algorithm is broken, there may be more than one header editor with
focused=true. Reset the focused flag before forwarding the mouse event
to the composer grid.

Signed-off-by: Robin Jarry <robin@jarry.cc>
Tested-by: Bence Ferdinandy <bence@ferdinandy.com>

widgets/compose.go

@@ -617,6 +617,12 @@ func (c *Composer) Event(event tcell.Event) bool {
 func (c *Composer) MouseEvent(localX int, localY int, event tcell.Event) {
 	c.Lock()
 	defer c.Unlock()
+	for _, e := range c.focusable {
+		he, ok := e.(*headerEditor)
+		if ok && he.focused {
+			he.focused = false
+		}
+	}
 	c.grid.MouseEvent(localX, localY, event)
 	for i, e := range c.focusable {
 		he, ok := e.(*headerEditor)