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>

core/overlay_layers.go

@@ -2,16 +2,16 @@ package core
 
 import (
 	"log/slog"
-	"time"
 
 	"github.com/egoist/mygo/ui"
 )
 
 // overlayLayers is the window's open MujicaUI layers, oldest first.
 type overlayLayers struct {
-	frame, prev time.Time
-	keys        []any
-	seen        []time.Time
+	root ui.Element // compared only: a new root means a new build pass
+	pass int
+	keys []any
+	seen []int
 }
 
 type overlayLayersKey struct{}
@@ -19,11 +19,12 @@ type overlayLayersKey struct{}
 // OverlayLayer marks the layer key open in this frame and reports whether it is on top.
 func OverlayLayer(c *ui.Context, key any) bool {
 	s := ui.Local(c.Root(), overlayLayersKey{}, func() overlayLayers { return overlayLayers{} })
-	if now := c.Now(); now != s.frame {
-		s.prev, s.frame = s.frame, now
+	if root := c.Root(); root != s.root {
+		s.root = root
+		s.pass++
 		n := 0
 		for i, k := range s.keys {
-			if !s.seen[i].Before(s.prev) { // drop layers the last frame did not build
+			if s.seen[i] >= s.pass-1 { // drop layers the last pass did not build
 				s.keys[n], s.seen[n] = k, s.seen[i]
 				n++
 			}
@@ -38,10 +39,10 @@ func OverlayLayer(c *ui.Context, key any) bool {
 		}
 	}
 	if at < 0 {
-		s.keys, s.seen = append(s.keys, key), append(s.seen, s.frame)
+		s.keys, s.seen = append(s.keys, key), append(s.seen, s.pass)
 		at = len(s.keys) - 1
 	}
-	s.seen[at] = s.frame
+	s.seen[at] = s.pass
 	return at == len(s.keys)-1
 }
 
@@ -76,7 +77,7 @@ type overlaySiteState struct {
 }
 
 // OverlaySite builds the invisible focus fallback at a layer's call site.
-func OverlaySite(c *ui.Context) (*ui.Element, *overlaySiteState) {
+func OverlaySite(c *ui.Context) (ui.Element, *overlaySiteState) {
 	site := ui.Box(c).Size(0, 0).FocusRing(false)
 	st := ui.Local(site, "mujica.overlay.site", func() overlaySiteState { return overlaySiteState{} })
 	st.holding = st.holding && site.FocusWithin()
@@ -89,12 +90,12 @@ func OverlaySite(c *ui.Context) (*ui.Element, *overlaySiteState) {
 }
 
 // Shown records whether the focus is in the open layer's panel.
-func (st *overlaySiteState) Shown(panel *ui.Element) {
-	st.focusIn, st.wait = panel != nil && panel.FocusWithin(), 0
+func (st *overlaySiteState) Shown(panel ui.Element) {
+	st.focusIn, st.wait = panel.Valid() && panel.FocusWithin(), 0
 }
 
 // Closed moves a lost focus to the site once MyGo has had its chance to restore it.
-func (st *overlaySiteState) Closed(c *ui.Context, site *ui.Element) {
+func (st *overlaySiteState) Closed(c *ui.Context, site ui.Element) {
 	if st.focusIn {
 		st.focusIn, st.wait = false, 2 // MyGo prunes the panel at the end of the next frame
 	}