PhetMenu shouldn't need to use getNextFocusable()

[zepumph]

While working on https://github.com/phetsims/phet-io/issues/1748, which took us to phetsims/scenery#1284 with @jessegreenberg, we realized that using document.activeElement most likely won't play nicely with PhET-iO. Furthermore, using PDOMUtils.getNextFocusable() in general for the PhetMenu and MenuItem seemed like a crutch, something that we don't really need to do.

@jessegreenberg if I misremember our conversation from last Friday, please fill in the gaps. I'd like to explore actively focusing the elements that we are supposed to be using. We can potentially use the snapshot-comparison tool to make sure that we don't have any regressions.

I will work on this as I work on phetsims/scenery#1284.

[jessegreenberg]

That is right, thanks. I don't know why PDOMUtils.getNextFocusable and PDOMUtils.getPreviousFocusable are being used here. Maybe this (untested) patch could work.

Index: js/PhetMenu.js
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- js/PhetMenu.js	(revision d428469a77688d6ff6a0da092e4447f55b07d25d)
+++ js/PhetMenu.js	(date 1632416499826)
@@ -426,6 +426,8 @@
 
         const firstItem = this.items[ 0 ];
         const lastItem = this.items[ this.items.length - 1 ];
+        const indexOfFocus = this.items.indexOf( FocusManager.pdomFocusedNode );
+        assert && assert( indexOfFocus > -1, 'how did we receive a keydown while focus is not in the menu?' );
 
         const key = KeyboardUtils.getEventCode( domEvent );
 
@@ -437,13 +439,13 @@
         if ( key === KeyboardUtils.KEY_DOWN_ARROW ) {
 
           // On down arrow, focus next item in the list, or wrap up to the first item if focus is at the end
-          const nextFocusable = lastItem.focused ? firstItem : PDOMUtils.getNextFocusable();
+          const nextFocusable = lastItem.focused ? firstItem : this.items[ indexOfFocus + 1 ];
           nextFocusable.focus();
         }
         else if ( key === KeyboardUtils.KEY_UP_ARROW ) {
 
           // On up arrow, focus previous item in the list, or wrap back to the last item if focus is on first item
-          const previousFocusable = firstItem.focused ? lastItem : PDOMUtils.getPreviousFocusable();
+          const previousFocusable = firstItem.focused ? lastItem : this.items[ indexOfFocus - 1 ];
           previousFocusable.focus();
         }
         else if ( key === KeyboardUtils.KEY_ESCAPE || key === KeyboardUtils.KEY_TAB ) {

Or maybe it could be even better getting rid of firstItem and lastItem since we are calculating the index anyway.

[zepumph]

This work pertains to phetsims/scenery#1298.