RelatedItemsFieldWidget: usability completely broken

[zopyx]

We have a slideshow portlet defines as

class ISlideshowPortlet(IPortletDataProvider):
    """Schema for a slideshow portlet.
    """

    header = TextLine(
        title=_(u"Header"),
        description=_(u"The header."),
        required=False,
        default=u"Slideshow")

    widget(
        'images',
        RelatedItemsFieldWidget,
        pattern_options={
            'selectableTypes': ['Image']
        }
    )

    images = RelationList(
        title=_(u"Images"),
        default=[],
        value_type=zope.schema.Choice(
            title=_("Image"),
            required=True,
            vocabulary='plone.app.vocabularies.Catalog'),
        required=False)

For the first time the widget opens the list with the contents below:

For the second time the content list is place above:

And for the third time the content list opens again below with the result that have of the contents list is hidden and can not be reached:

The UI approach is broken from ground up.

[fredvd]

@zopyx The issue is not exactly the same, but I encountered multiple problems with TinyMCE menu's disappaering last year, the common pattern here is the modal pattern from Mockup.

See these two issues:

• TinyMCE menu in RichText fields in tiled editor on modal renders erratic above/below the editor area plone.app.mosaic#425
• TinyMCE menu dropdowns in modal edit forms are displaced depending on PageScroll mockup#867

I only see the scroll position of the whole page slightly in your third screenshot. Can you confirm that the scroll position of the whole page is related to the shifting of the content list in the related items widget in the modal?