D29826: [KMainWindow] Invoke QIcon::setFallbackThemeName (later)

[Igor Poboiko]

poboiko created this revision.
poboiko added reviewers: aacid, mart, broulik.
Herald added a project: Frameworks.
poboiko requested review of this revision.

REVISION SUMMARY
  This is alternative approach to D22488: invoke QIcon::setFallbackThemeName a bit later <https://phabricator.kde.org/D22488> and commit 4214045 <https://phabricator.kde.org/R302:421404563e87a50d0af22eb8ca09866fd89a36cc> to KIconThemes.
  Okular (and most - if not all - KDE apps inherit KMainWindow, so KDE apps
  should have breeze icons). KMainWindow ctor should be early enough so no icons
  are yet loaded, but late enough so QGuiApplication is already inited.
  
  This should be followed by reverting commit 4214045 <https://phabricator.kde.org/R302:421404563e87a50d0af22eb8ca09866fd89a36cc> in KIconThemes.
  
  Original problem description (by @mart):
  invoking QIcon::setFallbackThemeName at QCoreApplication ctor
  with Q_COREAPP_STARTUP_FUNCTION breaks the internal status of
  QIconLoader as it instantiates it before the QPlatformTheme,
  but QIconLoader depends from QPlatformTheme to be already instantiated
  otherwise it won't load correctly, thus breaking icon loading
  in QtQuickControls2 styles, such as Material and Fusion
  see https://bugreports.qt.io/browse/QTBUG-74252
  
  CCBUG: 402172

TEST PLAN
  Don't have GTK3 QPA plugin, so cannot test it yet.
  I would appreciate if someone helped me with testing :)

REPOSITORY
  R263 KXmlGui

BRANCH
  icon-load (branched from master)

REVISION DETAIL
  https://phabricator.kde.org/D29826

AFFECTED FILES
  src/kmainwindow.cpp

To: poboiko, aacid, mart, broulik
Cc: mart, kde-frameworks-devel, LeGast00n, cblack, michaelh, ngraham, bruns
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