Gui: Fix document tree background rendering with overlay (Qt6)

This aims to fix rendering of tree view items in Qt6. While I don't
belive that this is a good way to fix this, I am worried that it is the
only way to do ir.

BC BREAK: This change introduces artificial QTreeView widget that can be
targeted using QSS and can be used in the delegate for painting background of
items. `QTreeView::item` would now be used to render background for the
whole row, while each cell can be targeted using `#DocumentTreeItems`
selector.

More details on implementation:
https://stackoverflow.com/questions/78414383/qt6-disable-drawing-of-default-background-for-qtreeview-items/78421604#78421604

Fixes: #13760

src/Gui/Tree.cpp

@@ -371,6 +371,21 @@ namespace Gui {
  */
 class TreeWidgetItemDelegate: public QStyledItemDelegate {
  typedef QStyledItemDelegate inherited;
+
+ // Beware, big scary hack incoming!
+ //
+ // This is artificial QTreeWidget that is not rendered and its sole goal is to be the source
+ // of style information that can be manipulated using QSS. From Qt6.5 tree branches also 
+ // have rendered background using ::item sub-control. Whole row also gets background from 
+ // the same sub-control. Only way to prevent this is to disable background of ::item,
+ // this however limits our ability to style tree items. As solution we create this widget
+ // that will be for painter to read information and draw proper backgrounds only when asked.
+ //
+ // More information: https://github.com/FreeCAD/FreeCAD/pull/13807
+ QTreeView *artificial;
+
+ QRect calculateItemRect(const QStyleOptionViewItem &option, const QModelIndex &index) const;
+
 public:
  explicit TreeWidgetItemDelegate(QObject* parent=nullptr);
 
@@ -389,6 +404,40 @@ class TreeWidgetItemDelegate: public QStyledItemDelegate {
 TreeWidgetItemDelegate::TreeWidgetItemDelegate(QObject* parent)
  : QStyledItemDelegate(parent)
 {
+ artificial = new QTreeView(qobject_cast<QWidget*>(parent));
+ artificial->setObjectName(QString::fromLatin1("DocumentTreeItems"));
+ artificial->setFixedSize(0, 0); // ensure that it does not render
+}
+
+
+QRect TreeWidgetItemDelegate::calculateItemRect(const QStyleOptionViewItem &option, const QModelIndex &index) const
+{
+ auto tree = static_cast<TreeWidget*>(parent());
+ auto style = tree->style();
+
+ QRect rect = option.rect;
+
+ const int margin = style->pixelMetric(QStyle::PM_FocusFrameHMargin, &option, artificial) + 1;
+
+ // 2 margin for text, 2 margin for decoration (icon) = 4 times margin
+ int width = 4 * margin 
+ + option.fontMetrics.boundingRect(option.text).width()
+ + option.decorationSize.width()
+ + TreeParams::getItemBackgroundPadding()
+ ;
+
+ if (TreeParams::getCheckBoxesSelection()) {
+ // another 2 margin for checkbox
+ width += 2 * margin 
+ + style->pixelMetric(QStyle::PM_IndicatorWidth)
+ + style->pixelMetric(QStyle::PM_LayoutHorizontalSpacing);
+ }
+
+ if (width < rect.width()) {
+ rect.setWidth(width);
+ }
+
+ return rect;
 }
 
 void TreeWidgetItemDelegate::paint(QPainter *painter,
@@ -397,59 +446,61 @@ void TreeWidgetItemDelegate::paint(QPainter *painter,
  QStyleOptionViewItem opt = option;
  initStyleOption(&opt, index);
 
- TreeWidget * tree = static_cast<TreeWidget*>(parent());
+ auto tree = static_cast<TreeWidget*>(parent());
  auto style = tree->style();
 
  // If the second column is not shown, we'll trim the color background when
  // rendering as transparent overlay.
  bool trimBG = TreeParams::getHideColumn();
- QRect rect = opt.rect;
 
  if (index.column() == 0) {
  if (tree->testAttribute(Qt::WA_NoSystemBackground)
  && (trimBG || (opt.backgroundBrush.style() == Qt::NoBrush
  && _TreeItemBackground.style() != Qt::NoBrush)))
  {
- const int margin = style->pixelMetric(QStyle::PM_FocusFrameHMargin, &option, tree) + 1;
- // 2 margin for text, 2 margin for decoration (icon)
- int width = 4*margin + opt.fontMetrics.boundingRect(opt.text).width()
- + opt.decorationSize.width() + TreeParams::getItemBackgroundPadding();
- if (TreeParams::getCheckBoxesSelection()) {
- // another 2 margin for checkbox
- width += 2*margin + style->pixelMetric(QStyle::PM_IndicatorWidth)
- + style->pixelMetric(QStyle::PM_LayoutHorizontalSpacing);
- }
- if (width < rect.width())
- rect.setWidth(width);
- if (trimBG) {
- rect.setWidth(rect.width() + 5);
- opt.rect = rect;
- if (opt.backgroundBrush.style() == Qt::NoBrush)
- painter->fillRect(rect, _TreeItemBackground);
- } else if (!opt.state.testFlag(QStyle::State_Selected))
+ QRect rect = calculateItemRect(option, index);
+
+ if (trimBG && opt.backgroundBrush.style() == Qt::NoBrush) {
+ painter->fillRect(rect, _TreeItemBackground);
+ } else if (!opt.state.testFlag(QStyle::State_Selected)) {
  painter->fillRect(rect, _TreeItemBackground);
+ }
  }
-
  }
 
- style->drawControl(QStyle::CE_ItemViewItem, &opt, painter, tree);
+ style->drawControl(QStyle::CE_ItemViewItem, &opt, painter, artificial);
 }
 
 void TreeWidgetItemDelegate::initStyleOption(QStyleOptionViewItem *option,
  const QModelIndex &index) const
 {
  inherited::initStyleOption(option, index);
 
- TreeWidget * tree = static_cast<TreeWidget*>(parent());
- QTreeWidgetItem * item = tree->itemFromIndex(index);
- if (!item || item->type() != TreeWidget::ObjectType)
+ auto tree = static_cast<TreeWidget*>(parent());
+ auto item = tree->itemFromIndex(index);
+
+ if (!item) {
  return;
+ }
+
+ auto mousePos = option->widget->mapFromGlobal(QCursor::pos());
+ auto isHovered = option->rect.contains(mousePos);
+ if (!isHovered) {
+ option->state &= ~QStyle::State_MouseOver;
+ }
+
+ QSize size = option->icon.actualSize(QSize(0xffff, 0xffff));
 
- QSize size;
- size = option->icon.actualSize(QSize(0xffff, 0xffff));
- if (size.height())
- option->decorationSize = QSize(size.width()*TreeWidget::iconSize()/size.height(),
- TreeWidget::iconSize());
+ if (size.height() > 0) {
+ option->decorationSize = QSize(
+ size.width() * TreeWidget::iconSize() / size.height(),
+ TreeWidget::iconSize()
+ );
+ }
+
+ if (TreeParams::getHideColumn()) {
+ option->rect = calculateItemRect(*option, index);
+ }
 }
 
 QWidget* TreeWidgetItemDelegate::createEditor(
@@ -509,6 +560,7 @@ TreeWidget::TreeWidget(const char* name, QWidget* parent)
  this->setDragDropMode(QTreeWidget::InternalMove);
  this->setColumnCount(2);
  this->setItemDelegate(new TreeWidgetItemDelegate(this));
+ this->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
 
  this->showHiddenAction = new QAction(this);
  this->showHiddenAction->setCheckable(true);
@@ -586,7 +638,7 @@ TreeWidget::TreeWidget(const char* name, QWidget* parent)
  //NOLINTEND
 
  setupResizableColumn(this);
- this->header()->setStretchLastSection(false);
+ this->header()->setStretchLastSection(true);
  QObject::connect(this->header(), &QHeaderView::sectionResized, [](int idx, int, int newSize) {
  if (idx)
  TreeParams::setColumnSize2(newSize);