[LyX/master] PrefColors dialog shows transparent swatches correctly

[Koji Yokota]

commit 5d497f745bdc64d6858fba4e585df33f6a65d786
Author: Koji Yokota <yok...@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
-- 
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