Created attachment 118056 [details]
Application menu button fallback to the app's icon

SUMMARY
Aurorae theme engine allows using SVG files to render window decorations, but there's a lack of support for "application menu" button.

When "application menu" button is added to the title bar, its icon will instead fallback to the app's icon.

If this file is indeed the one that loads Aurorae themes, https://phabricator.kde.org/source/kwin/browse/master/plugins/kdecorations/aurorae/src/lib/auroraetheme.cpp$111, then yeah, there doesn't seem to be any line in it whatsoever to load SVG file for "application menu" button.

This documentation also does not say anything about it, https://phabricator.kde.org/source/kwin/browse/master/plugins/kdecorations/aurorae/theme-description.

There used to be a bug report somewhat related to this, https://bugs.kde.org/show_bug.cgi?id=375862, but the proposed solution was only to allow adding the icon programmatically (which Plastik theme does).

STEPS TO REPRODUCE
1. Install an SVG-based Aurorae theme.
2. Go to System Settings > Application Style > Window Decorations.
3. Apply the said theme in Theme tab.
4. Go to Buttons tab, then add Application menu button somewhere in the title bar.

OBSERVED RESULT
Application menu button fallback to the app's icon instead of trying to load an SVG icon from the theme.
Then again there's no support for it anyway.

EXPECTED RESULT
Application menu button supports SVG file.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: KDE neon 5.14
KDE Plasma Version: 5.15.0
KDE Frameworks Version: 5.55.0
Qt Version: 5.12.0

ADDITIONAL INFORMATION
Relevant issue in an SVG-based theme's Github repository: https://github.com/PapirusDevelopmentTeam/materia-kde/issues/37.