Add support for mouse hover events

event.go

@@ -293,6 +293,7 @@ func (window *window) handleButtonRelease (
 	window.updateModifiers(buttonEvent.State)
 	window.updateMousePosition(buttonEvent.EventX, buttonEvent.EventY)
 	dragging := window.drags[buttonEvent.Detail]
+	window.drags[buttonEvent.Detail] = nil
 	
 	if dragging != nil {
 		dragging.handleMouseUp(input.Button(buttonEvent.Detail))
@@ -314,11 +315,14 @@ func (window *window) handleMotionNotify (
 	for _, child := range window.drags {
 		if child == nil { continue }
 		child.handleMouseMove()
+		window.hover(child)
 		handled = true
 	}
 
 	if !handled {
-		window.boxUnder(image.Pt(x, y)).handleMouseMove()
+		underneath := window.boxUnder(image.Pt(x, y))
+		underneath.handleMouseMove()
+		window.hover(underneath)
 	}
 }
 

system.go

@@ -56,8 +56,8 @@ type anyBox interface {
 	// handleDndEnter   ()
 	// handleDndLeave   ()
 	// handleDndDrop    (data.Data)
-	// handleMouseEnter ()
+	handleMouseEnter ()
-	// handleMouseLeave ()
+	handleMouseLeave ()
 	handleMouseMove  ()
 	handleMouseDown  (input.Button)
 	handleMouseUp    (input.Button)
@@ -119,15 +119,31 @@ func (window *window) focus (box anyBox) {
 	window.focused = box
 	
 	if previous != nil {
+		// FIXME why are we invalidating draw here
 		window.invalidateDraw(previous)
 		previous.handleFocusLeave()
 	}
 	if box != nil && box.canBeFocused() {
+		// FIXME why are we invalidating draw here
 		window.invalidateDraw(box)
 		box.handleFocusEnter()
 	}
 }
 
+func (window *window) hover (box anyBox) {
+	if window.hovered == box { return }
+	
+	previous := window.hovered
+	window.hovered = box
+	
+	if previous != nil {
+		previous.handleMouseLeave()
+	}
+	if box != nil {
+		box.handleMouseEnter()
+	}
+}
+
 func (window *window) anyFocused () bool {
 	return window.focused != nil
 }

window.go

@@ -44,6 +44,7 @@ type window struct {
 
 	root    anyBox
 	focused anyBox
+	hovered anyBox
 
 	needDraw   boxSet
 	needLayout boxSet