[Markup] Drag decoration issue on controls with markups

[mknauer]

Copy of Bugzilla bug 579671

When you activate drag and drop on controls labelled with HTML markup (TableItem, TreeItem, ...), hover decoration on dragging is the inner html content of markup (as text), which is troublesome.

It should be at least the same behaviour as other drag operations in RAP, which display icon + text under cursor while dragging.

See https://www.eclipse.org/forums/index.php/t/1110563/

Ideally and in general we should be able to customize the hover decoration when dragging some controls (feel free to open another request for that).

[mknauer]

Implemented and fixed with commit 4645849.