D7089: [Breeze Style] Fix flickering during KPageTreeView animation

[Kai Uwe Broulik]

broulik created this revision.
Restricted Application added a project: Plasma.
Restricted Application added a subscriber: plasma-devel.

REVISION SUMMARY
  QTreeView animates expanding/collapsing of branches. It renders the widget into a temporary pixmap for speed which is then animated. The pixmap's background, however, is unconditionally filled with the widget's palette's base color which is different from the window background color onto which we paint when side panel frames are disabled.
  
  This leads to flickering as the background is different during the animation. Depending on the colorscheme can be quite noticeable (e.g. Honeycomb).

TEST PLAN
  In Kate's settings dialog, expanded and collapsed some trees, background is consistent now. Tried with various colorschemes and background was always good and tree lines and text remained readable. Also verified that with side panel frame enabled it still works and that regular QTreeView isn't affected.
  
  Before
  F3829093: Screenshot_20170803_114937.png <https://phabricator.kde.org/F3829093>
  After
  F3829092: Screenshot_20170803_114718.png <https://phabricator.kde.org/F3829092>

REPOSITORY
  R31 Breeze

REVISION DETAIL
  https://phabricator.kde.org/D7089

AFFECTED FILES
  kstyle/breezestyle.cpp

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