The font used by Qt4 applications is always set to sans-serif 9pt, no matter what I select in system settings. Even if I set the font with qtconfig-qt4, it is reset on login. 

Steps to Reproduce: 
1) In system settings, set system font to MSYH 10pt (for example). 
2) Open a Qt4 application such as VLC. The font used is sans-serif 9pt.
3) In qtconfig-qt4, set font to MSYH 10pt.
4) Open the Qt4 application. The font used is MSYH 10pt.
5) Logout and login. Open the Qt4 application. The font used is again sans-serif 9pt.

Actual Results:
Qt4 applications use sans-serif 9pt at 2) and 5).

Expected Results:
Qt4 applications use MSYH 10pt, as configured in system settings.


Additional Information:

The config file that Qt4 applications use is ~/.config/Trolltech.conf. On login, kcminit (kcminit style) writes KDE5 settings to this file. In plasma-desktop/kcms/krdb/krdb.cpp, function applyQtSettings:

    settings.setValue(QStringLiteral("/qt/font"), QFontDatabase::systemFont(QFontDatabase::GeneralFont).toString());

Qt's default font is written to Trolltech.conf, which is supposed to be the font set in system settings on KDE5. However, this is launched by kcminit, which has plasma-workspace/startkde/kcminit/main.cpp, function kdemain:

    QGuiApplication::setDesktopSettingsAware(false);
    QGuiApplication app(argc, argv);

Without desktopSettingsAware, Qt's default font is forced to be sans-serif 9pt, which is then written to Trolltech.conf and causes this bug. 

Suggested fix: use KDE config to get the font instead of QFontDatabase::systemFont.


--- a/krdb.cpp	2017-03-21 22:00:06.000000000 +0800
+++ b/krdb.cpp	2017-03-30 15:44:20.103183732 +0800
@@ -178,10 +178,13 @@
 static void applyQtSettings( KSharedConfigPtr kglobalcfg, QSettings& settings )
 {
   /* export font settings */
-  settings.setValue(QStringLiteral("/qt/font"), QFontDatabase::systemFont(QFontDatabase::GeneralFont).toString());
+  KConfigGroup kdeCfgGroup(kglobalcfg, "General");
+  QString font = kdeCfgGroup.readEntry("font", QString());
+  settings.setValue(QStringLiteral("/qt/font"), !font.isEmpty() ? font :
+    QFontDatabase::systemFont(QFontDatabase::GeneralFont).toString());
 
   /* export effects settings */
-  KConfigGroup kdeCfgGroup(kglobalcfg, "General");
+  
   bool effectsEnabled = kdeCfgGroup.readEntry("EffectsEnabled", false);
   bool fadeMenus = kdeCfgGroup.readEntry("EffectFadeMenu", false);
   bool fadeTooltips = kdeCfgGroup.readEntry("EffectFadeTooltip", false);


Or remove that QGuiApplication::setDesktopSettingsAware(false).