Let the Collections and Projects panels follow the palette

Both panels forced a light palette on themselves (white rows, black
text, their own selection blue) so that element previews, which are
black line art drawn for the white sheet, would stay visible on a dark
desktop (bugtracker 335). On a dark palette the two docks were the
only white windows left.

The forced palettes are gone. Element previews are now kept as drawn,
on a transparent background, and adapted where they are shown:
ElementPreviewDelegate, installed on the collection tree, hands the
view a copy with its lightness inverted when the palette is dark
(QET::Palette::forPalette), so black ink becomes the palette's light
gray while colored icons such as folders stay as they are; the drag
pixmap is adapted the same way. A light palette shows the previews
untouched. This fixes bugtracker 335 on every dark desktop rather than
masking it with a white panel.

The preview cache stored the old white-sheet pictures; it records the
format now and drops a cache written before this change once. The
amber "show this directory" highlight sets black text so it reads on
both palettes. The Projects panel only shows icons from the icon
theme, which has a dark variant, so nothing else changes there.

tests/qttest/tst_qetpalette: the line-art rule tells ink from color;
inversion keeps hue and alpha; a preview reads at 3:1 on the Base
color of both palettes; in a tree on the dark palette the delegate
inverts a line-art icon and leaves a colored one alone.

Fixes #945.

sources/ElementsCollection/elementpreviewdelegate.cpp

@@ -0,0 +1,54 @@
+/*
+	Copyright 2006-2026 The QElectroTech Team
+	This file is part of QElectroTech.
+
+	QElectroTech is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 2 of the License, or
+	(at your option) any later version.
+
+	QElectroTech is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with QElectroTech.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#include "elementpreviewdelegate.h"
+
+#include <QWidget>
+
+#include "../qetpalette.h"
+
+/**
+	@brief ElementPreviewDelegate::initStyleOption
+	After the base class has filled the option from the model, replace
+	the icon with one that reads on a dark palette. The adapted icon is
+	built from the pixmap the view is about to draw, at the view's
+	decoration size and device pixel ratio.
+*/
+void ElementPreviewDelegate::initStyleOption(QStyleOptionViewItem *option,
+                                             const QModelIndex &index) const
+{
+	QStyledItemDelegate::initStyleOption(option, index);
+	if (option->icon.isNull() || !QET::Palette::isDark(option->palette))
+		return;
+
+	const qint64 key = option->icon.cacheKey();
+	const auto it = m_dark_icons.constFind(key);
+	if (it != m_dark_icons.constEnd())
+	{
+		option->icon = *it;
+		return;
+	}
+
+	const qreal dpr = option->widget ? option->widget->devicePixelRatio() : 1.0;
+	const QPixmap source = option->icon.pixmap(option->decorationSize, dpr);
+	const QPixmap adapted = QET::Palette::forPalette(source, option->palette);
+	QIcon icon = option->icon;
+	if (adapted.cacheKey() != source.cacheKey())
+		icon = QIcon(adapted);
+	m_dark_icons.insert(key, icon);
+	option->icon = icon;
+}

sources/ElementsCollection/elementpreviewdelegate.h

@@ -0,0 +1,50 @@
+/*
+	Copyright 2006-2026 The QElectroTech Team
+	This file is part of QElectroTech.
+
+	QElectroTech is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 2 of the License, or
+	(at your option) any later version.
+
+	QElectroTech is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with QElectroTech.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#ifndef ELEMENTPREVIEWDELEGATE_H
+#define ELEMENTPREVIEWDELEGATE_H
+
+#include <QHash>
+#include <QIcon>
+#include <QStyledItemDelegate>
+
+/**
+	@brief The ElementPreviewDelegate class
+	Draws the items of an element collection tree with icons that read on
+	the current palette. Element previews are black line art drawn for a
+	white sheet; on a dark palette this delegate hands the view the same
+	picture with its lightness inverted (QET::Palette::forPalette), so the
+	ink is light on the dark row. Colored icons, such as folders, are left
+	alone, and nothing changes on a light palette. Adapted icons are kept
+	per source icon, so a repaint costs a hash lookup.
+*/
+class ElementPreviewDelegate : public QStyledItemDelegate
+{
+	Q_OBJECT
+
+	public:
+		using QStyledItemDelegate::QStyledItemDelegate;
+
+	protected:
+		void initStyleOption(QStyleOptionViewItem *option,
+		                     const QModelIndex &index) const override;
+
+	private:
+		mutable QHash<qint64, QIcon> m_dark_icons;
+};
+
+#endif

sources/ElementsCollection/elementscollectionwidget.cpp

@@ -725,7 +725,10 @@ void ElementsCollectionWidget::showThisDir()
 		ElementCollectionItem *eci =
 				elementCollectionItemForIndex(m_showed_index);
 		if (eci)
+		{
 			eci->setBackground(QBrush());
+			eci->setForeground(QBrush());
+		}
 	}
 
 	m_showed_index = m_index_at_context_menu;
@@ -736,7 +739,11 @@ void ElementsCollectionWidget::showThisDir()
 		ElementCollectionItem *eci =
 				elementCollectionItemForIndex(m_showed_index);
 		if (eci)
+		{
+			// Amber under black, whatever the palette's text color.
 			eci->setBackground(QBrush(QColor(255, 204, 0, 255)));
+			eci->setForeground(QBrush(Qt::black));
+		}
 		search();
 	}
 	else
@@ -755,7 +762,10 @@ void ElementsCollectionWidget::resetShowThisDir()
 		ElementCollectionItem *eci = elementCollectionItemForIndex(
 					m_showed_index);
 		if (eci)
+		{
 			eci->setBackground(QBrush());
+			eci->setForeground(QBrush());
+		}
 	}
 
 	m_showed_index = QModelIndex();

sources/ElementsCollection/elementstreeview.cpp

@@ -16,6 +16,8 @@
 	along with QElectroTech.  If not, see <http://www.gnu.org/licenses/>.
 */
 #include "elementstreeview.h"
+#include "elementpreviewdelegate.h"
+#include "../qetpalette.h"
 
 #include "../factory/elementfactory.h"
 #include "../qetgraphicsitem/element.h"
@@ -41,24 +43,11 @@ static int MAX_DND_PIXMAP_HEIGHT = 375;
 ElementsTreeView::ElementsTreeView(QWidget *parent) :
 	QTreeView(parent)
 {
-	// force du noir sur une alternance de blanc (comme le schema) et de gris
-	// clair, avec du blanc sur bleu pas trop fonce pour la selection
-	//
-	// Element icons are rendered with colors read directly from each .elmt
-	// file (almost always black linework, matching printed-schematic
-	// convention) onto a transparent background -- so this view must keep
-	// a light background regardless of the OS/desktop theme, or the icons
-	// become invisible on dark themes. QAbstractItemView paints its rows
-	// using the viewport's palette, not the view's own, so the palette
-	// must be applied to both to actually take effect under every style.
-	QPalette qp = palette();
-	qp.setColor(QPalette::Text,            Qt::black);
-	qp.setColor(QPalette::Base,            Qt::white);
-	qp.setColor(QPalette::AlternateBase,   QColor("#e8e8e8"));
-	qp.setColor(QPalette::Highlight,       QColor("#678db2"));
-	qp.setColor(QPalette::HighlightedText, Qt::black);
-	setPalette(qp);
-	viewport()->setPalette(qp);
+	// Rows follow the application palette. Element previews are black
+	// line art drawn for a white sheet; ElementPreviewDelegate adapts them
+	// to a dark palette, so this view no longer has to force a light one
+	// (bugtracker #335).
+	setItemDelegate(new ElementPreviewDelegate(this));
 }
 
 /**
@@ -220,7 +209,7 @@ void ElementsTreeView::startElementDrag(const ElementsLocation &location)
 			&elmt_creation_state));
 		if (elmt_creation_state) { return; }
 
-		QPixmap elmt_pixmap(temp_elmt->pixmap());
+		QPixmap elmt_pixmap(QET::Palette::forPalette(temp_elmt->pixmap(), palette()));
 		QPoint elmt_hotspot(temp_elmt->hotspot());
 
 			//Adjust the size of the pixmap if he is too big