[PATCH] print: fix black screen on macOS arm64 after PDF export

On macOS arm64 (Apple Silicon, Sequoia), exporting a PDF via
QPrintPreviewWidget leaves a black screen with only the mouse cursor
visible.  Cmd+Tab restores the display; the exported PDF itself is
correct and clickable cross-reference links work fine.

Root cause
----------
requestPaint() is a slot connected to QPrintPreviewWidget::paintRequested.
Inside this slot the code was calling painter.end() manually, then
pdfConvertUriToGoTo().  On macOS arm64 the Qt5 paint cycle backed by
Metal/CALayer is asynchronous: closing the QPainter from *within* the
paintRequested slot interrupts the compositor before it has flushed the
backing store.  The window goes black and never repaints because the
close() that follows immediately destroys it.

On x86_64 / older macOS (raster/CoreGraphics backend) the paint cycle is
synchronous, so the same code happened to work.

Fix
---
1. Remove the manual painter.end() and pdfConvertUriToGoTo() call from
   requestPaint().  The QPainter is stack-allocated; it destructs normally
   when the slot returns, which is the correct moment to flush the PDF.

2. In print(), capture the output file name before m_preview->print(),
   then defer both pdfConvertUriToGoTo() and this->close() to the next
   event-loop iteration via QTimer::singleShot(0, ...).  This gives the
   Metal compositor one full event-loop turn to finish compositing the
   backing store before the window is torn down.

The fix is a no-op on all other platforms: QTimer::singleShot(0) posts
an event that fires in the very next iteration, so there is no perceptible
delay.

Tested
------
- macOS Sequoia 15.x, Apple M-series, Qt 5.15.x (arm64): black screen gone
- macOS 10.15 x86_64 VM, Qt 5.15.x: no regression
- Linux/Debian Qt 5.15.x: no regression
- PDF cross-reference links and GoTo/FitR destinations: unaffected

Fixes: black screen after PDF export on macOS arm64

sources/print/projectprintwindow.cpp

@@ -47,6 +47,7 @@
 #include <QFile>
 #include <QRegularExpression>
 #include <QMap>
+#include <QTimer>
 #include <QVector>
 
 /**
@@ -495,13 +496,12 @@ void ProjectPrintWindow::requestPaint()
 		printDiagram(diagram, ui->m_fit_in_page_cb->isChecked(), &painter, m_printer, diagramPageMap);
 	}
 
-	if (pdfExport) {
-		painter.end();   // flush & close the PDF file on disk
-		// Convert URI link annotations into native internal GoTo/FitR actions:
-		// cross-references then jump inside the document (no new viewer
-		// instance) and frame the target element.
-		pdfConvertUriToGoTo(m_printer->outputFileName());
-	}
+	// Note: do NOT call painter.end() or pdfConvertUriToGoTo() here.
+	// We are inside the paintRequested slot: the QPrintPreviewWidget still
+	// owns the paint cycle.  On macOS arm64 (Metal/CALayer compositor),
+	// closing the QPainter manually inside this slot leaves the backing
+	// store in an undefined state, producing a black screen after export.
+	// pdfConvertUriToGoTo() is deferred to print() via QTimer::singleShot(0).
 }
 
 /**
@@ -1218,9 +1218,29 @@ void ProjectPrintWindow::on_m_uncheck_all_clicked()
 
 void ProjectPrintWindow::print()
 {
-	m_preview->print();
+	const bool isPdf = (m_printer->outputFormat() == QPrinter::PdfFormat);
+	const QString pdfFile = isPdf ? m_printer->outputFileName() : QString();
+
+	m_preview->print();   // triggers requestPaint() synchronously; painter
+	                      // is created/destroyed inside that call
+
 	savePageSetupForCurrentPrinter();
-	this->close();
+
+	if (isPdf && !pdfFile.isEmpty()) {
+		// Defer post-processing and window close to the next event-loop
+		// iteration.  This lets the macOS arm64 Metal compositor finish
+		// compositing the backing store before the window is destroyed,
+		// which prevents the black screen observed on Apple Silicon under
+		// macOS Sequoia (QPrintPreviewWidget + CALayer timing issue).
+		QTimer::singleShot(0, this, [this, pdfFile]() {
+			// Convert URI link annotations into native internal GoTo/FitR
+			// actions so cross-references jump inside the document.
+			pdfConvertUriToGoTo(pdfFile);
+			this->close();
+		});
+	} else {
+		this->close();
+	}
 }
 
 void ProjectPrintWindow::on_m_date_cb_userDateChanged(const QDate &date)