[LyX/master] Pinch to zoom fix

[Stephan Witt]

commit 7134ebd7938ee2f2e075d53b83a346b168a622c1
Author: Daniel Ramoeller <d.lyx at web.de>
Date:   Wed Jan 5 15:27:46 2022 +0100

    Pinch to zoom fix
    
    Amendment to fix for #12434.
---
 src/frontends/qt/GuiView.cpp |    5 ++++-
 src/frontends/qt/GuiView.h   |    3 +++
 2 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/src/frontends/qt/GuiView.cpp b/src/frontends/qt/GuiView.cpp
index 6cd97f5..91e8229 100644
--- a/src/frontends/qt/GuiView.cpp
+++ b/src/frontends/qt/GuiView.cpp
@@ -1632,8 +1632,11 @@ bool GuiView::event(QEvent * e)
 		if (gp) {
 			QPinchGesture *pinch = static_cast<QPinchGesture *>(gp);
 			QPinchGesture::ChangeFlags changeFlags = pinch->changeFlags();
+			if (pinch->state() == Qt::GestureStarted) {
+				initialZoom_ = lyxrc.currentZoom;
+			}
 			if (changeFlags & QPinchGesture::ScaleFactorChanged) {
-				qreal factor = lyxrc.currentZoom*pinch->scaleFactor();
+				qreal factor = initialZoom_ * pinch->totalScaleFactor();
 				zoomValueChanged(factor);
 			}
 		}
diff --git a/src/frontends/qt/GuiView.h b/src/frontends/qt/GuiView.h
index 145a1f2..9ad3610 100644
--- a/src/frontends/qt/GuiView.h
+++ b/src/frontends/qt/GuiView.h
@@ -526,6 +526,9 @@ private:
 
 	// developer mode
 	bool devel_mode_;
+
+	// initial zoom for pinch gesture
+	int initialZoom_;
 };