widget: make Button trigger unfocus when tapped.

Fixes #5107.

widget/button.go

@@ -191,6 +191,12 @@ func (b *Button) Tapped(*fyne.PointEvent) {
 	b.tapAnimation()
 	b.Refresh()
 
+	if !b.focused {
+		// Grab the focus to unfocus any previous widget, then release it.
+		focusIfNotMobile(b.super())
+		b.FocusLost()
+	}
+
 	if onTapped := b.OnTapped; onTapped != nil {
 		onTapped()
 	}

widget/button_internal_test.go

@@ -235,3 +235,47 @@ func TestButtonRenderer_TapAnimation(t *testing.T) {
 	button.tapAnim.Tick(0.5)
 	test.AssertImageMatches(t, "button/tap_animation.png", w.Canvas().Capture())
 }
+
+func TestButton_TappedFocus(t *testing.T) {
+	test.NewApp()
+	entry := NewEntry()
+	button := NewButton("ok", nil)
+	w := test.NewTempWindow(t, newCont(entry, button))
+
+	c := w.Canvas()
+	c.Focus(entry)
+	assert.True(t, entry.focused, "entry is not focused")
+
+	// NB. we cannot use test.Tap(button) as it handles focus,
+	// and we want to specifically test Button's handling of focus.
+	button.Tapped(&fyne.PointEvent{})
+	assert.False(t, entry.focused, "entry still has focus")
+	assert.False(t, button.focused, "button still has focus")
+}
+
+type cont struct {
+	BaseWidget
+	objects []fyne.CanvasObject
+}
+
+type contRenderer struct {
+	cont *cont
+}
+
+var _ fyne.CanvasObject = (*cont)(nil)
+
+func newCont(objects ...fyne.CanvasObject) *cont {
+	c := &cont{objects: objects}
+	c.ExtendBaseWidget(c)
+	return c
+}
+
+func (c *cont) CreateRenderer() fyne.WidgetRenderer {
+	return &contRenderer{cont: c}
+}
+
+func (r *contRenderer) Destroy()                     {}
+func (r *contRenderer) Layout(fyne.Size)             {}
+func (r *contRenderer) MinSize() fyne.Size           { return fyne.NewSize(10, 10) }
+func (r *contRenderer) Objects() []fyne.CanvasObject { return r.cont.objects }
+func (r *contRenderer) Refresh()                     {}

widget/check.go

@@ -380,7 +380,7 @@ func (c *checkRenderer) updateFocusIndicator(th fyne.Theme, v fyne.ThemeVariant)
 }
 
 func focusIfNotMobile(w fyne.Widget) {
-	if !fyne.CurrentDevice().IsMobile() {
+	if w != nil && !fyne.CurrentDevice().IsMobile() {
 		if c := fyne.CurrentApp().Driver().CanvasForObject(w); c != nil {
 			c.Focus(w.(fyne.Focusable))
 		}