QETApp::useSystemPalette(true) installed a one-rule application stylesheet
whose only declaration was invalid CSS:
QAbstractScrollArea#mdiarea {
background-color -> setPalette(initial_palette_);
}
That is not a CSS declaration but a note-to-self, committed in e6c32bc0
("Background set to use System Palette", 2014) when a hardcoded
background-color:#D5D2D1;
was replaced with a reminder to derive the color from the palette instead.
Qt's CSS parser silently skips invalid declarations, and at the time the
same rule still carried valid background-image/-repeat/-position
properties, so the block kept working and nothing looked wrong. Those
properties were dropped later, leaving a rule with no valid declarations
at all.
The rule has therefore styled nothing for some time. It is not harmless
though: a non-empty application stylesheet wraps every widget in
QStyleSheetStyle, which overrides per-widget QWidget::setStyle(). QET
does not currently call QWidget::setStyle() anywhere, so nothing is
visibly broken today, but it blocks that API for future work — it was
found while prototyping a palette-based dark mode (see #553).
Replace it with an explicit setStyleSheet(QString()). The behavior of the
"use system colors" branch is unchanged: it already dropped whatever
style.css had loaded (by overwriting it with the inert rule), and the
qApp->setPalette(initial_palette_) call on the line above is what actually
supplies the system colors — which is what the 2014 note was asking for.
The style.css path (use == false) is untouched.
Reported by DieterMayerOSS in #553.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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