Please build (release) the example below on Windows and apply windeployqt on the executable with windeployqt --qmldir ....

The resulting application distribution will run but:

[1504] Default ui delegate ToolTip.qml can not be found.

On trying to upload a file (Add Files button on the example website):

[1504] Default ui delegate FilePicker.qml can not be found.

It seems windeployqt completely misses this components:

• QtQuick.Dialogs
• QtQuick.Controls

For WebEngine itself the ControlsDelegate part is also missing:

qml/QtWebEngine/ControlsDelegate/qtwebenginequickdelegatesplugin.dll
qml/QtWebEngine/ControlsDelegate/FilePicker.qml. (<< why?)
qml/QtWebEngine/ControlsDelegate/ToolTip.qml
...
Qt6WebEngineQuickDelegatesQml.dll

I also wonder why the .qml sources are required at all. For every other plugin the qml bytecode or whatever is included is sufficient. It's only in the context of the ControlsDelegate that the qml files are required.

Minimal WebView example (based on WebEngineView, but the same problem exists when using the WebView WebEngine backend):

import QtQuick
import QtWebEngine

Window {
  width: 640
  height: 480
  visible: true
  title: qsTr("Hello World")

  WebEngineView {
    anchors.fill: parent
    url: "https://blueimp.github.io/jQuery-File-Upload/"
  }
}