Also draw the arrow dark when in dark theme with highlight

src/calibre/gui2/progress_indicator/CalibreStyle.cpp

@@ -7,6 +7,7 @@
 
 #include "QProgressIndicator.h"
 #include <QPainter>
+#include <QPixmapCache>
 #include <QtWidgets/QStyle>
 #include <QtWidgets/QApplication>
 #include <QDebug>
@@ -61,6 +62,82 @@ static inline QByteArray detectDesktopEnvironment()
     return QByteArrayLiteral("UNKNOWN");
 }
 
+#ifdef Q_OS_DARWIN
+static const qreal qstyleBaseDpi = 72;
+#else
+static const qreal qstyleBaseDpi = 96;
+#endif
+
+qreal dpiScaled(qreal value, qreal dpi)
+{
+    return value * dpi / qstyleBaseDpi;
+}
+
+static QPixmap styleCachePixmap(const QSize &size)
+{
+    const qreal pixelRatio = qApp->devicePixelRatio();
+    QPixmap cachePixmap = QPixmap(size * pixelRatio);
+    cachePixmap.setDevicePixelRatio(pixelRatio);
+    return cachePixmap;
+}
+
+
+static void draw_arrow(Qt::ArrowType type, QPainter *painter, const QStyleOption *option, const QRect &rect, const QColor &color)
+{
+    if (rect.isEmpty())
+        return;
+
+    const qreal dpi = option->fontMetrics.fontDpi();
+    const int arrowWidth = int(dpiScaled(14, dpi));
+    const int arrowHeight = int(dpiScaled(8, dpi));
+
+    const int arrowMax = qMin(arrowHeight, arrowWidth);
+    const int rectMax = qMin(rect.height(), rect.width());
+    const int size = qMin(arrowMax, rectMax);
+
+    QPixmap cachePixmap;
+    QString cacheKey = QString("calibre-tree-view-arrow-%1-%2-%3").arg((unsigned long)color.rgba()).arg((unsigned long)type).arg(size);
+    if (!QPixmapCache::find(cacheKey, &cachePixmap)) {
+        cachePixmap = styleCachePixmap(rect.size());
+        cachePixmap.fill(Qt::transparent);
+        QPainter cachePainter(&cachePixmap);
+
+        QRectF arrowRect;
+        arrowRect.setWidth(size);
+        arrowRect.setHeight(arrowHeight * size / arrowWidth);
+        if (type == Qt::LeftArrow || type == Qt::RightArrow)
+            arrowRect = arrowRect.transposed();
+        arrowRect.moveTo((rect.width() - arrowRect.width()) / 2.0,
+                         (rect.height() - arrowRect.height()) / 2.0);
+
+        QPolygonF triangle;
+        triangle.reserve(3);
+        switch (type) {
+        case Qt::DownArrow:
+            triangle << arrowRect.topLeft() << arrowRect.topRight() << QPointF(arrowRect.center().x(), arrowRect.bottom());
+            break;
+        case Qt::RightArrow:
+            triangle << arrowRect.topLeft() << arrowRect.bottomLeft() << QPointF(arrowRect.right(), arrowRect.center().y());
+            break;
+        case Qt::LeftArrow:
+            triangle << arrowRect.topRight() << arrowRect.bottomRight() << QPointF(arrowRect.left(), arrowRect.center().y());
+            break;
+        default:
+            triangle << arrowRect.bottomLeft() << arrowRect.bottomRight() << QPointF(arrowRect.center().x(), arrowRect.top());
+            break;
+        }
+
+        cachePainter.setPen(Qt::NoPen);
+        cachePainter.setBrush(color);
+        cachePainter.setRenderHint(QPainter::Antialiasing);
+        cachePainter.drawPolygon(triangle);
+
+        QPixmapCache::insert(cacheKey, cachePixmap);
+    }
+
+    painter->drawPixmap(rect, cachePixmap);
+}
+
 
 CalibreStyle::CalibreStyle(int transient_scroller) : QProxyStyle(QString::fromUtf8("Fusion")), transient_scroller(transient_scroller) {
     setObjectName(QString("calibre"));
@@ -212,6 +289,14 @@ void CalibreStyle::drawPrimitive(PrimitiveElement element, const QStyleOption *
             }
             break; // }}}
 
+        case PE_IndicatorBranch:  // {{{
+            if (option->state & State_MouseOver && option->state & State_Children && widget && widget->property("hovered_item_is_highlighted").toBool() && is_color_dark(option->palette.color(QPalette::Window))) {;
+                if (option->rect.width() <= 1 || option->rect.height() <= 1) return;
+                Qt::ArrowType arrow = Qt::ArrowType::DownArrow;
+                if (!(option->state & State_Open)) arrow = Qt::ArrowType::RightArrow;
+                draw_arrow(arrow, painter, option, option->rect, QColor(Qt::black));
+                return;
+            } break;  // }}}
         case PE_IndicatorToolBarSeparator:  // {{{
             // Make toolbar separators stand out a bit more in dark themes
             {

src/calibre/gui2/tag_browser/view.py

@@ -241,6 +241,7 @@ class TagsView(QTreeView):  # {{{
         '''.replace('PAD', str(gprefs['tag_browser_item_padding'])) + (
             '' if gprefs['tag_browser_old_look'] else stylish_tb) + QApplication.instance().palette_manager.tree_view_hover_style()
         )
+        self.setProperty('hovered_item_is_highlighted', True)
 
     def set_look_and_feel(self, first=False):
         self.set_style_sheet()

src/calibre/gui2/viewer/toc.py

@@ -89,6 +89,7 @@ class TOCView(QTreeView):
             }
 
         ''' + QApplication.instance().palette_manager.tree_view_hover_style())
+        self.setProperty('hovered_item_is_highlighted', True)
 
     def mouseMoveEvent(self, ev):
         if self.indexAt(ev.pos()).isValid():