Upgrade MyGo to v0.5.5

MyGo v0.5.5 returns ui.Element as a checked value handle instead of a
pointer. Migrated with `mygo migrate-ui`, then fixed what the codemod
leaves to review:

- Element nil checks become .Valid(); absent elements are ui.Element{}.
- Input handlers and paint callbacks run outside the build pass, where
  elements expire and Context methods are inert: read IsDisabled into a
  local during the build, keep a ui.Handle for Focus, use c.Services()
  for clipboard access, create ui.Local state during the build.
- Add core.AutoFocus: v0.5.5 records an overlay's opener after its
  content builds, so same-frame autofocus lost focus restore on close.
  It focuses one frame later; cascader rows defer their focus likewise.
- core.OverlayLayer identifies build passes by the root element, not
  c.Now(), which repeats across passes and under the Tester clock.
- Context menus survive the opening press counted as PressedOutside;
  split button tooltip no longer reorders its open menu; disabled
  menubar rows pass presses to the panel; Form defers blur-started
  async checks so the growing message no longer moves Submit away
  from the release; Combobox drops its disabled copy now that MyGo
  blocks typing under a disabled wrapper (canary flipped to a guard).
- The Tester clock is virtual: tests use Advance/Now instead of
  time.Sleep/time.Now and read element state inside the view. The web
  mirror advances the clock to wall time before each frame and input.
- AGENTS.md and docs describe the value Element API and the new pin.

Snapshot goldens are darwin-only and were not re-verified here.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

overlay/context_menu.go

@@ -13,13 +13,13 @@ type ContextMenuOptions struct {
 
 // contextMenuState is where the menu opened, relative to the target.
 type contextMenuState struct {
-	open bool
-	x, y float32
+	open, fresh bool // fresh: opened by a press the menu has not yet seen
+	x, y        float32
 }
 
 // ContextMenu gives the container target an in-window menu at the pointer or below the focus.
-func ContextMenu(c *ui.Context, target *ui.Element, opts ContextMenuOptions, build func(m *PopupMenu)) *ui.Element {
-	if target == nil {
+func ContextMenu(c *ui.Context, target ui.Element, opts ContextMenuOptions, build func(m *PopupMenu)) ui.Element {
+	if !target.Valid() {
 		panic("mujica: ContextMenu needs a target")
 	}
 	if build == nil {
@@ -34,7 +34,7 @@ func ContextMenu(c *ui.Context, target *ui.Element, opts ContextMenuOptions, bui
 		if ev.Kind != ui.InputPointerDown || ev.Button != 1 {
 			return false
 		}
-		st.open, st.x, st.y = true, ev.X, ev.Y
+		st.open, st.fresh, st.x, st.y = true, true, ev.X, ev.Y
 		slog.Debug("mujica: context_menu open", "by", "pointer")
 		return true
 	})
@@ -45,10 +45,15 @@ func ContextMenu(c *ui.Context, target *ui.Element, opts ContextMenuOptions, bui
 	if !st.open {
 		return target
 	}
-	var anchor *ui.Element
+	var anchor ui.Element
 	target.Children(func() {
-		anchor = ui.Box(c).Key("mujica.contextmenu.anchor").Absolute().Left(st.x).Top(st.y).Size(1, 1).PassThrough()
+		anchor = ui.Box(c.Key("mujica.contextmenu.anchor")).Absolute().Left(st.x).Top(st.y).Size(1, 1).PassThrough()
 	})
-	PopupMenuOpen(c, anchor, &st.open, build)
+	panel := PopupMenuOpen(c, anchor, &st.open, build)
+	// The opening press fell outside the new panel; it must not dismiss it.
+	if st.fresh && panel.Valid() && panel.Dismissed() {
+		st.open = true
+	}
+	st.fresh = false
 	return target
 }