D20140: [GridDelegate] Fix gaps in corners of thumbnailArea highlight

[Noah Davis]

ndavis edited the test plan for this revision.

REPOSITORY
  R296 KDeclarative

REVISION DETAIL
  https://phabricator.kde.org/D20140

To: ndavis, #plasma, #vdg
Cc: kde-frameworks-devel, michaelh, ngraham, bruns