[LyX/master] PrefColors dialog shows transparent swatches correctly

[Koji Yokota]

commit 5d497f745bdc64d6858fba4e585df33f6a65d786
Author: Koji Yokota <yokota at lyx.org>
Date:   Thu May 14 19:19:51 2026 +0900

    PrefColors dialog shows transparent swatches correctly
---
 src/frontends/qt/GuiPrefs.cpp | 35 +++++++++++++++++++++++++++++++++--
 src/frontends/qt/GuiPrefs.h   |  3 +++
 2 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/src/frontends/qt/GuiPrefs.cpp b/src/frontends/qt/GuiPrefs.cpp
index 8b89f7ec34..4cbce38a0b 100644
--- a/src/frontends/qt/GuiPrefs.cpp
+++ b/src/frontends/qt/GuiPrefs.cpp
@@ -4720,6 +4720,26 @@ ColorSwatchDelegate::ColorSwatchDelegate(QObject *parent)
     : QStyledItemDelegate(parent), button_(new QPushButton)
 {
 	pane_ = static_cast<PrefColors*>(parent);
+
+	// construct the background image of swatches
+	bg_pixmap_ = new QPixmap(pane_->swatch_width_, pane_->swatch_height_);
+	bg_pixmap_->fill(Qt::white);
+	QPainter painter(bg_pixmap_);
+	painter.setRenderHint(QPainter::Antialiasing);
+	QPolygon polygon;
+	polygon << QPoint(pane_->swatch_width_, 0)
+	        << QPoint(0, pane_->swatch_height_)
+	        << QPoint(pane_->swatch_width_, pane_->swatch_height_);
+	painter.setPen(QColor(Qt::black));
+	painter.setBrush(QColor(Qt::black));
+	painter.drawPolygon(polygon);
+	painter.end();
+}
+
+
+ColorSwatchDelegate::~ColorSwatchDelegate()
+{
+	delete bg_pixmap_;
 }
 
 
@@ -4743,9 +4763,10 @@ void ColorSwatchDelegate::paint(QPainter *painter,
 		QColor color =
 		        pane_->colorsTV_model_.data(index, Qt::DecorationRole).
 		        value<QColor>();
-		if (index.flags().testFlag(Qt::ItemIsEnabled))
+		if (index.flags().testFlag(Qt::ItemIsEnabled)) {
 			pixmap.fill(color);
-		else
+			pixmap = mergePixmaps(&pixmap, bg_pixmap_);
+		} else
 			pixmap.fill(Qt::transparent);
 		style->drawItemPixmap(painter, opt.rect, Qt::AlignCenter, pixmap);
 	} else {
@@ -4755,6 +4776,16 @@ void ColorSwatchDelegate::paint(QPainter *painter,
 }
 
 
+QPixmap ColorSwatchDelegate::mergePixmaps(QPixmap const *fg, QPixmap const *bg) const
+{
+	// make a copy to leave bg untouched
+	QPixmap merged(*bg);
+	QPainter painter(&merged);
+	painter.drawPixmap(0, 0, *bg);
+	painter.drawPixmap(0, 0, *fg);
+	return merged;
+}
+
 } // namespace frontend
 } // namespace lyx
 
diff --git a/src/frontends/qt/GuiPrefs.h b/src/frontends/qt/GuiPrefs.h
index ab67f69ebc..441adc81c3 100644
--- a/src/frontends/qt/GuiPrefs.h
+++ b/src/frontends/qt/GuiPrefs.h
@@ -735,12 +735,15 @@ class ColorSwatchDelegate : public QStyledItemDelegate
 
 public:
 	ColorSwatchDelegate(QObject *parent = nullptr);
+	~ColorSwatchDelegate();
 
 	void paint(QPainter *painter, const QStyleOptionViewItem &option,
 	           const QModelIndex &index) const override;
 
 private:
+	QPixmap mergePixmaps(QPixmap const * fg, QPixmap const * bg) const;
 	PrefColors* pane_;
+	QPixmap* bg_pixmap_;
 	QFont font_;
 
 	// To give impression that a button is pressed, flip the direction of