Handle entering FocusWrap from outside

When FocusWrap was used, but the previous focus was outside of the wrapped
element, the browser would try to focus the boundary element, causing us
to focus the last element instead. This is fixed by checking the relatedTarget
of the event, and specially handling focus coming from outside.

Fixes #3636.

assets/js/phoenix_live_view/hooks.js

@@ -47,8 +47,26 @@ let Hooks = {
     mounted(){
       this.focusStart = this.el.firstElementChild
       this.focusEnd = this.el.lastElementChild
-      this.focusStart.addEventListener("focus", () => ARIA.focusLast(this.el))
-      this.focusEnd.addEventListener("focus", () => ARIA.focusFirst(this.el))
+      this.focusStart.addEventListener("focus", (e) => {
+        if(!e.relatedTarget || !this.el.contains(e.relatedTarget)){ 
+          // Handle focus entering from outside (e.g. Tab when body is focused)
+          // https://github.com/phoenixframework/phoenix_live_view/issues/3636
+          const nextFocus = e.target.nextElementSibling
+          ARIA.attemptFocus(nextFocus) || ARIA.focusFirst(nextFocus)
+        } else {
+          ARIA.focusLast(this.el)
+        }
+      })
+      this.focusEnd.addEventListener("focus", (e) => {
+        if(!e.relatedTarget || !this.el.contains(e.relatedTarget)){ 
+          // Handle focus entering from outside (e.g. Shift+Tab when body is focused)
+          // https://github.com/phoenixframework/phoenix_live_view/issues/3636
+          const nextFocus = e.target.previousElementSibling
+          ARIA.attemptFocus(nextFocus) || ARIA.focusLast(nextFocus)
+        } else {
+          ARIA.focusFirst(this.el)
+        }
+      })
       this.el.addEventListener("phx:show-end", () => this.el.focus())
       if(window.getComputedStyle(this.el).display !== "none"){
         ARIA.focusFirst(this.el)