Shrink the Selection properties panel again when nothing is selected

#1533 stopped the panel shrinking to its title bar when the selection
had nothing to edit, so that it would keep the height the user gave it
(#1439). Users want the room back for the Collections and Auto numbering
panels while nothing is selected (#1566).

The panel's layout is back to QLayout::SetMaximumSize, so the empty
panel shrinks as before. Just before its last editor goes, the panel
keeps the heights of every docked panel in its area; when an editor
comes back it gives them back with QMainWindow::resizeDocks, the call a
separator drag makes. Giving only this panel its height back fell 35 px
short in the test: the neighbours had been split again from their size
hints and gave up only part of the room.

Test: tst_selectionpanelheight now checks, twice over, that the empty
panel shrinks and that selecting an element again brings back the height
the user gave it. Without the restore the panel comes back at 470 px
instead of 560; on master the empty panel does not shrink.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sources/PropertiesEditor/propertieseditordockwidget.cpp

@@ -19,6 +19,9 @@
 #include "ui_propertieseditordockwidget.h"
 #include "propertieseditorwidget.h"
 
+#include <QMainWindow>
+#include <QTimer>
+
 /**
 	@brief PropertiesEditorDockWidget::PropertiesEditorDockWidget
 	Constructor
@@ -48,6 +51,9 @@ PropertiesEditorDockWidget::~PropertiesEditorDockWidget()
 */
 void PropertiesEditorDockWidget::clear()
 {
+	if (!m_editor_list.isEmpty())
+		rememberHeight();
+
 	foreach (PropertiesEditorWidget *editor, m_editor_list)
 	{
 		m_editor_list.removeOne(editor);
@@ -94,8 +100,11 @@ bool PropertiesEditorDockWidget::addEditor(PropertiesEditorWidget *editor,
 	if (!editor) return false;
 	if (m_editor_list.contains(editor)) return true;
 
+	const bool was_empty = m_editor_list.isEmpty();
 	ui -> m_main_vlayout -> insertWidget(index, editor);
 	m_editor_list << editor;
+	if (was_empty)
+		restoreHeight();
 	return true;
 }
 
@@ -118,9 +127,84 @@ QList<PropertiesEditorWidget *> PropertiesEditorDockWidget::editors() const
 */
 bool PropertiesEditorDockWidget::removeEditor(PropertiesEditorWidget *editor)
 {
+	if (m_editor_list.size() == 1 && m_editor_list.first() == editor)
+		rememberHeight();
+
 	bool result = m_editor_list.removeOne(editor);
 	if (result)
 		ui -> m_main_vlayout -> removeWidget(editor);
 
 	return result;
 }
+
+/**
+	@brief PropertiesEditorDockWidget::rememberHeight
+	The dock shrinks to its title bar while it has no editor, and the
+	other docks of its area take the room. Keep the heights the docks
+	of the area have now, the heights the user gave them, to give them
+	back when an editor comes back.
+*/
+void PropertiesEditorDockWidget::rememberHeight()
+{
+		// Still the shrunk height: the last editor came and went
+		// before the heights were given back
+	if (m_restore_pending || isFloating() || !isVisible())
+		return;
+	auto main_window = qobject_cast<QMainWindow *>(parentWidget());
+	if (!main_window)
+		return;
+
+	m_area_docks.clear();
+	m_area_heights.clear();
+	const Qt::DockWidgetArea area = main_window->dockWidgetArea(this);
+	for (QDockWidget *dock : main_window->findChildren<QDockWidget *>())
+	{
+		if (dock->parentWidget() == main_window && !dock->isFloating()
+			&& dock->isVisible()
+			&& main_window->dockWidgetArea(dock) == area)
+		{
+			m_area_docks << dock;
+			m_area_heights << dock->height();
+		}
+	}
+}
+
+/**
+	@brief PropertiesEditorDockWidget::restoreHeight
+	Give the docks of the area back the heights they had before this
+	dock shrank. Without this the area is split again from the docks'
+	size hints, and giving this dock alone its height back takes the
+	room only from its neighbours.
+*/
+void PropertiesEditorDockWidget::restoreHeight()
+{
+	if (m_area_docks.isEmpty() || m_restore_pending)
+		return;
+
+	m_restore_pending = true;
+		// Once the layout has given the dock its maximum height back
+	QTimer::singleShot(0, this, [this]() {
+		m_restore_pending = false;
+		auto main_window = qobject_cast<QMainWindow *>(parentWidget());
+		if (!main_window || isFloating() || !isVisible()
+			|| m_editor_list.isEmpty())
+			return;
+
+		QList<QDockWidget *> docks;
+		QList<int> heights;
+		const Qt::DockWidgetArea area = main_window->dockWidgetArea(this);
+		for (int i = 0 ; i < m_area_docks.size() ; ++i)
+		{
+			QDockWidget *dock = m_area_docks.at(i);
+				// Closed, moved or deleted since
+			if (dock && !dock->isFloating() && dock->isVisible()
+				&& main_window->dockWidgetArea(dock) == area)
+			{
+				docks << dock;
+				heights << m_area_heights.at(i);
+			}
+		}
+		if (docks.contains(this))
+			main_window->resizeDocks(docks, heights, Qt::Vertical);
+	});
+}