With the attached reproducer and a minimal .pro such as:

QT += core widgets webenginewidgets

TARGET = testbrowser
TEMPLATE = app

SOURCES += main.cpp

with Qt 6.3, the focus rectangle appears after 2s as expected:

and the printed output is:

JS focused:  QVariant(QString, "TEXTAREA")
QApp focused:  QtWebEngineCore::RenderWidgetHostViewQtDelegateWidget(0x55bd630ed580)

With Qt 6.4, however, the JS element is focused, and keyboard input lands there, but no focus rectangle or flashing cursor appears:

and the output shows that some QQuickWidget rather than the RenderWidgetHostViewQtDelegateWidget has focus:

JS focused:  QVariant(QString, "TEXTAREA")
QApp focused:  QQuickWidget(0x559a5cee8220)

When clicking the web page (anywhere outside of the textarea) before the 2s passed, then things work as expected, however.