D5323: Correctly handle when a new primary screen displaces the old

[Marco Martin]

mart added a reviewer: subdiff.

REPOSITORY
  R120 Plasma Workspace

REVISION DETAIL
  https://phabricator.kde.org/D5323

To: mart, #plasma, broulik, sebas, subdiff
Cc: plasma-devel, progwolff, lesliezhai, ali-mohamed, jensreuterberg, abetts, 
sebas, apol