Nixpkgs: No icons in KDE applications running outside Plasma

[ttuegel]

Description

KDE applications do not have any icons when running outside the Plasma 5 desktop environment. Qt applications in general do not appear with the correct theme.

Technical details

• Nixpkgs version: e8880627e8

[ttuegel]

This is not a bug; this is how Qt 5 is supposed to work.

Outside of Plasma, one must manually set the environment variable QT_QPA_PLATFORMTHEME environment variable to select a platform theme. Setting kde will mimic the Plasma theme. Setting qt5ct will allow using the qt5ct package to configure the theme.

[olejorgenb]

Would be nice if that variable somehow got a sane default though. Okular for instance works ok without it (lots of icons missing, but not that big a deal actually). BUT - startup time is affected due to how icon lookup works: Without QT_QPA_PLATFORMTHEME set okular makes 400k+ stat calls looking for icons. With the variable set this is reduced to 9k and startup time is at least cut in half...

EDIT: seems it's set if config.programs.qt5ct.enable is true.

[qknight]

@ttuegel i've tried to set it with qt5ct but i see this error message:

nix-shell -p qt5ct

inside this shell:

qt5ct 
This application failed to start because it could not find or load the Qt platform plugin "xcb"
in "".

Reinstalling the application may fix this problem.
Aborted

nixos-version 
18.03pre124015.f59a0f7f1a6 (Impala)

[chrissound]

Do I need any additional packages for icons to show? Does anyone know which ones?

Output of QT_QPA_PLATFORMTHEME=kde dolphin:

kf5.kio.core: Refilling KProtocolInfoFactory cache in the hope to find "stash"
Icon theme "breeze" not found.
Icon theme "breeze" not found.
Error: standard icon theme "breeze" not found!
kf5.kio.core: Refilling KProtocolInfoFactory cache in the hope to find "mtp"
kf5.kio.core: Refilling KProtocolInfoFactory cache in the hope to find "mtp"
kf5.kio.core: Refilling KProtocolInfoFactory cache in the hope to find "mtp"
org.kde.dolphin: Ignore KIO url: QUrl("timeline:/today")
org.kde.dolphin: Ignore KIO url: QUrl("timeline:/yesterday")
org.kde.dolphin: Ignore KIO url: QUrl("timeline:/thismonth")
org.kde.dolphin: Ignore KIO url: QUrl("timeline:/lastmonth")
org.kde.dolphin: Ignore KIO url: QUrl("search:/documents")
org.kde.dolphin: Ignore KIO url: QUrl("search:/images")
org.kde.dolphin: Ignore KIO url: QUrl("search:/audio")
org.kde.dolphin: Ignore KIO url: QUrl("search:/videos")
qt.accessibility.core: Cannot create accessible child interface for object:  PlacesView(0x19b89c0)  index:  20
QPixmap::scaled: Pixmap is a null pixmap
QPixmap::scaled: Pixmap is a null pixmap
QPixmap::scaled: Pixmap is a null pixmap