From 2316435f2fd2da28c70e1b251b852d3bf6d8011a Mon Sep 17 00:00:00 2001
From: Jean-Marc Lasgouttes <lasgouttes@lyx.org>
Date: Sat, 25 Nov 2017 12:31:11 +0100
Subject: [PATCH] Use a backing store on macOS

Qt on macOS does not respect the Qt::WA_OpaquePaintEvent attribute and
clears the widget backing store at each update. Therefore, we use our
own backing store in this case.

This restores a simplified version of the code that was removed at 24c29908.
---
 src/frontends/qt4/GuiWorkArea.cpp       | 11 +++++--
 src/frontends/qt4/GuiWorkArea_Private.h | 44 +++++++++++++++++++++++++
 2 files changed, 53 insertions(+), 2 deletions(-)

diff --git a/src/frontends/qt4/GuiWorkArea.cpp b/src/frontends/qt4/GuiWorkArea.cpp
index 264933b082..0df7b65c78 100644
--- a/src/frontends/qt4/GuiWorkArea.cpp
+++ b/src/frontends/qt4/GuiWorkArea.cpp
@@ -304,6 +304,7 @@ void GuiWorkArea::init()
 			generateSyntheticMouseEvent();
 		});
 
+	d->resetScreen();
 	// With Qt4.5 a mouse event will happen before the first paint event
 	// so make sure that the buffer view has an up to date metrics.
 	d->buffer_view_->resize(viewport()->width(), viewport()->height());
@@ -1231,12 +1232,15 @@ void GuiWorkArea::paintEvent(QPaintEvent * ev)
 	// LYXERR(Debug::PAINTING, "paintEvent begin: x: " << rc.x()
 	//	<< " y: " << rc.y() << " w: " << rc.width() << " h: " << rc.height());
 
-	if (d->need_resize_ || pixelRatio() != d->last_pixel_ratio_)
+	if (d->need_resize_ || pixelRatio() != d->last_pixel_ratio_) {
+		d->resetScreen();
 		d->resizeBufferView();
+	}
 
 	d->last_pixel_ratio_ = pixelRatio();
 
-	GuiPainter pain(viewport(), pixelRatio());
+	GuiPainter pain(d->screenDevice(), pixelRatio());
+
 	d->buffer_view_->draw(pain, d->caret_visible_);
 
 	// The preedit text, if needed
@@ -1245,6 +1249,9 @@ void GuiWorkArea::paintEvent(QPaintEvent * ev)
 	// and the caret
 	if (d->caret_visible_)
 		d->caret_->draw(pain);
+
+	d->updateScreen(ev->rect());
+
 	ev->accept();
 }
 
diff --git a/src/frontends/qt4/GuiWorkArea_Private.h b/src/frontends/qt4/GuiWorkArea_Private.h
index de20397c71..83012fa99f 100644
--- a/src/frontends/qt4/GuiWorkArea_Private.h
+++ b/src/frontends/qt4/GuiWorkArea_Private.h
@@ -20,6 +20,14 @@
 #include <QMouseEvent>
 #include <QTimer>
 
+#ifdef Q_OS_MAC
+/* Qt on macOS does not respect the Qt::WA_OpaquePaintEvent attribute
+ * and resets the widget backing store at each update. Therefore, we
+ * use our own backing store in this case */
+#define LYX_BACKINGSTORE 1
+#include <QPainter>
+#endif
+
 namespace lyx {
 
 class Buffer;
@@ -99,6 +107,38 @@ struct GuiWorkArea::Private
 
 	void paintPreeditText(GuiPainter & pain);
 
+	void resetScreen() {
+#ifdef LYX_BACKINGSTORE
+		int const pr = p->pixelRatio();
+		screen_ = QImage(static_cast<int>(pr * p->viewport()->width()),
+		                 static_cast<int>(pr * p->viewport()->height()),
+		                 QImage::Format_ARGB32_Premultiplied);
+#  if QT_VERSION >= 0x050000
+		screen_.setDevicePixelRatio(pr);
+#  endif
+#endif
+	}
+
+	QPaintDevice * screenDevice() {
+#ifdef LYX_BACKINGSTORE
+		return &screen_;
+#else
+		return p->viewport();
+#endif
+	}
+
+#ifdef LYX_BACKINGSTORE
+	void updateScreen(QRectF const & rc) {
+		QPainter qpain(p->viewport());
+		double const pr = p->pixelRatio();
+		QRectF const rcs = QRectF(rc.x() * pr, rc.y() * pr,
+		                          rc.width() * pr, rc.height() * pr);
+		qpain.drawImage(rc, screen_, rcs);
+	}
+#else
+	void updateScreen(QRectF const & ) {}
+#endif
+
 	///
 	GuiWorkArea * p;
 	///
@@ -106,6 +146,10 @@ struct GuiWorkArea::Private
 	///
 	GuiView * lyx_view_;
 
+#ifdef LYX_BACKINGSTORE
+	///
+	QImage screen_;
+#endif
 	///
 	CaretWidget * caret_;
 	/// is the caret currently displayed
-- 
2.39.2