Touch events on the right and bottom edge of the screen are lost

The touch events on the right and bottom edge of the screen are lost.

This is due to an incorrect conversion of QPointF to QPoint inside QGuiApplicationPrivate::processTouchEvent.

Example

Consider a touch event with with screen position of QPointF(1919.5, 660.617) on the screen of size 1920x1080. This point when handled by the QGuiApplicationPrivate::processTouchEvent method will be passed to QGuiApplication::topLevelAt by the following code:

window = QGuiApplication::topLevelAt(touchPoint.screenPos().toPoint());

This code will call the toPoint method converting the touch location to QPoint(1920, 661). The test performed in QGuiApplication::topLevelAt against the screen size represented by the QRect(0, 0, 1920x1080) will then subsequently fail.

Fix

The following diff to qtbase fixes the issue:

diff --git a/src/gui/kernel/qguiapplication.cpp b/src/gui/kernel/qguiapplication.cpp
index d466ca6..d1ba1f5 100644
--- a/src/gui/kernel/qguiapplication.cpp
+++ b/src/gui/kernel/qguiapplication.cpp
@@ -2695,7 +2695,7 @@ void QGuiApplicationPrivate::processTouchEvent(QWindowSystemInterfacePrivate::To
             if (!w) {
                 // determine which window this event will go to
                 if (!window)
-                    window = QGuiApplication::topLevelAt(touchPoint.screenPos().toPoint());
+                    window = QGuiApplication::topLevelAt(QPoint((int)touchPoint.screenPos().x(), (int)touchPoint.screenPos().y()));
                 if (!window)
                     continue;
                 w = window;

Remarks

There are many places in QGuiApplicationPrivate that rely on the QPointF to QPoint transformation using the toPoint method. This includes mouse and tablet event handlers. I suspect this issue to be even more prevalent in the code.