TinyMCE menu in RichText fields in tiled editor on modal renders erratic above/below the editor area

[fredvd]

TinyMCE widgets on a tiled edit modal have their menu bar dropdown menu's displaced. At first I though it was somehow related to the location of the tile on the mosaic editor. Lower on the page seems to give more displacement, but not always. Further experimenting shows it's dependent on the scroll position of the mosaic editor page before you open the modal.

The menu's of 'inline' RichText tiles like Field - Text and Structure - RichText work fine, it's only happening in the modal edit forms.

It's a bit difficult to reproduce in core plone/mosaic because we have no out of the box tiles that have RichText in their schema. For testing I've added collective.tiles.advancedstatic to my plone.app.mosaic buildout. Testing this with test-5.1.coredev.cfg, but it's also happening with Mosaic 2.1.1 and Plone 5.1.3-pending.

If you place an advanced static tile in the middle of a filled mosaic editor page with some richt text structures above and below the advanced static tile you can open the edit modal in different pagescroll positions.

Visual demontration. So with this mosaic layout:

The tinymce richttext in the modal works ok:

But when I open the same tile edit modal with the page scrolled down:

The menu drop downs are displaced.