Add popovers

A popover is a special UI element which can be layered over the rest of
the UI (i.e. it is painted last) and can fall anywhere on the screen,
not just with the bounds of its parent's viewport/context. With these
special abilities comes the restriction that only one popover may be
visible on screen at once.

Popovers are requested from the UI context passed to Draw calls and
specify the anchor point and the desired dimensions. The popover is then
fit to the available space and placed relative to the anchor point.

lib/ui/ui.go

@@ -11,6 +11,7 @@ type UI struct {
 	exit    atomic.Value // bool
 	ctx     *Context
 	screen  tcell.Screen
+	popover *Popover
 
 	tcEvents chan tcell.Event
 	invalid  int32 // access via atomic
@@ -34,11 +35,11 @@ func Initialize(content DrawableInteractiveBeeper) (*UI, error) {
 
 	state := UI{
 		Content: content,
-		ctx:     NewContext(width, height, screen),
 		screen:  screen,
 
 		tcEvents: make(chan tcell.Event, 10),
 	}
+	state.ctx = NewContext(width, height, screen, state.onPopover)
 
 	state.exit.Store(false)
 	go func() {
@@ -57,6 +58,10 @@ func Initialize(content DrawableInteractiveBeeper) (*UI, error) {
 	return &state, nil
 }
 
+func (state *UI) onPopover(p *Popover) {
+	state.popover = p
+}
+
 func (state *UI) ShouldExit() bool {
 	return state.exit.Load().(bool)
 }
@@ -78,17 +83,32 @@ func (state *UI) Tick() bool {
 		case *tcell.EventResize:
 			state.screen.Clear()
 			width, height := event.Size()
-			state.ctx = NewContext(width, height, state.screen)
+			state.ctx = NewContext(width, height, state.screen, state.onPopover)
 			state.Content.Invalidate()
 		}
-		state.Content.Event(event)
+		// if we have a popover, and it can handle the event, it does so
+		if state.popover == nil || !state.popover.Event(event) {
+			// otherwise, we send the event to the main content
+			state.Content.Event(event)
+		}
 		more = true
 	default:
 	}
 
 	wasInvalid := atomic.SwapInt32(&state.invalid, 0)
 	if wasInvalid != 0 {
+		if state.popover != nil {
+			// if the previous frame had a popover, rerender the entire display
+			state.Content.Invalidate()
+			atomic.StoreInt32(&state.invalid, 0)
+		}
+		// reset popover for the next Draw
+		state.popover = nil
 		state.Content.Draw(state.ctx)
+		if state.popover != nil {
+			// if the Draw resulted in a popover, draw it
+			state.popover.Draw(state.ctx)
+		}
 		state.screen.Show()
 		more = true
 	}