Folio-wide crosshair under the mouse (Display menu)

A new option in the Display menu, off by default and kept in the
settings: two thin lines cross the whole folio, inside its border,
through the grid point under the mouse, so that one sees at a glance
whether symbols and terminals are in line, also while dragging them.

They are drawn by the view in drawForeground(), on screen only: no item
is added to the folio, so nothing of them is saved, exported or
printed. Only the strips the lines leave and enter are repainted, and
they go when the mouse leaves the view.

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

sources/diagramview.cpp

@@ -47,6 +47,7 @@
 #include "utils/conductorcreator.h"
 #include "undocommand/addgraphicsobjectcommand.h"
 #include "diagram.h"
+#include <QSettings>
 #include "foliogrid.h"
 #include "diagramcontexttoolbar.h"
 #include "imagedrop.h"
@@ -1040,6 +1041,7 @@ void DiagramView::mouseMoveEvent(QMouseEvent *e)
 {
 	m_last_mouse_pos = e->pos();
 	setToolTip(tr("X: %1 Y: %2").arg(e->pos().x()).arg(e->pos().y()));
+	moveCrosshair(e->pos());
 	if (m_event_interface && m_event_interface->mouseMoveEvent(e)) return;
 
 	if (m_gesture_tracking)
@@ -1840,6 +1842,117 @@ void DiagramView::setCellLinesShown(bool shown)
 	viewport()->update();
 }
 
+namespace {
+		/// The setting, read once and then kept here for every view
+	int s_crosshair = -1;
+}
+
+/**
+	@brief DiagramView::crosshairShown
+	@return true when the folio-wide crosshair follows the mouse: two thin
+	lines across the whole folio, through the grid point under the mouse,
+	to see at a glance whether symbols and terminals are in line. Drawn
+	on screen only, never exported nor printed.
+*/
+bool DiagramView::crosshairShown()
+{
+	if (s_crosshair < 0) {
+		QSettings settings;
+		s_crosshair = settings.value(QStringLiteral("diagrameditor/crosshair"), false).toBool();
+	}
+	return s_crosshair;
+}
+
+/**
+	@brief DiagramView::setCrosshairShown
+	@param shown : show the folio-wide crosshair in every view; kept in
+	the settings. The views repaint themselves at the next mouse move.
+*/
+void DiagramView::setCrosshairShown(bool shown)
+{
+	s_crosshair = shown;
+	QSettings settings;
+	settings.setValue(QStringLiteral("diagrameditor/crosshair"), shown);
+}
+
+/**
+	@brief DiagramView::crosshairArea
+	@return the part of the viewport the two lines through @a scene_pos
+	cover, a little wider than the lines
+*/
+QRect DiagramView::crosshairArea(const QPointF &scene_pos) const
+{
+	const QPoint p = mapFromScene(scene_pos);
+	const QRect all = viewport()->rect();
+	return QRect(all.left(), p.y() - 2, all.width(), 5)
+			| QRect(p.x() - 2, all.top(), 5, all.height());
+}
+
+/**
+	@brief DiagramView::moveCrosshair
+	Follow the mouse with the crosshair, on the grid, repainting only the
+	lines it leaves and the lines it draws.
+	@param viewport_pos : the mouse, in viewport coordinates
+*/
+void DiagramView::moveCrosshair(const QPoint &viewport_pos)
+{
+	if (!crosshairShown()) {
+		hideCrosshair();
+		return;
+	}
+	const QPointF pos = Diagram::snapToGrid(mapToScene(viewport_pos));
+	if (m_crosshair_visible && pos == m_crosshair_pos)
+		return;
+	if (m_crosshair_visible)
+		viewport()->update(crosshairArea(m_crosshair_pos));
+	m_crosshair_pos = pos;
+	m_crosshair_visible = true;
+	viewport()->update(crosshairArea(m_crosshair_pos));
+}
+
+/**
+	@brief DiagramView::hideCrosshair
+	Take the crosshair away, when the mouse leaves the view or the
+	setting is switched off.
+*/
+void DiagramView::hideCrosshair()
+{
+	if (!m_crosshair_visible)
+		return;
+	m_crosshair_visible = false;
+	viewport()->update(crosshairArea(m_crosshair_pos));
+}
+
+/**
+	@brief DiagramView::drawForeground
+	Over everything else, the folio-wide crosshair, inside the folio's
+	border only.
+	@param painter
+	@param rect
+*/
+void DiagramView::drawForeground(QPainter *painter, const QRectF &rect)
+{
+	PaletteGraphicsView::drawForeground(painter, rect);
+	if (!m_crosshair_visible || !crosshairShown() || !m_diagram)
+		return;
+
+	const QRectF area = m_diagram->border_and_titleblock.insideBorderRect();
+	if (!area.contains(m_crosshair_pos))
+		return;
+
+	QPainter *p = scenePainter(painter);
+	p->save();
+	QPen pen(Diagram::background_color == Qt::darkGray ? QColor(Qt::lightGray)
+													   : QColor(0, 120, 215));
+	pen.setCosmetic(true);
+	pen.setWidthF(0);
+	p->setPen(pen);
+	p->setRenderHint(QPainter::Antialiasing, false);
+	p->drawLine(QLineF(area.left(), m_crosshair_pos.y(), area.right(), m_crosshair_pos.y()));
+	p->drawLine(QLineF(m_crosshair_pos.x(), area.top(), m_crosshair_pos.x(), area.bottom()));
+	p->restore();
+}
+
 /**
 	@brief DiagramView::drawBackground
 	Reimplemented from PaletteGraphicsView: over the folio background, the
@@ -1976,6 +2089,9 @@ bool DiagramView::viewportEvent(QEvent *event)
 	if (event->type() == QEvent::Resize) {
 		placeCellRulers();
 	}
+	if (event->type() == QEvent::Leave) {
+		hideCrosshair();
+	}
 	return PaletteGraphicsView::viewportEvent(event);
 }