qpa: improve API to support DnDs from other processes

... instead of using a hack of directly accessing QGuiApplication
members.

The current QPA API was bad for two reasons:

1) It expects platform plugin authors to know about
internals of Qt Gui, particularly that QGuiApplication
uses QGuiApplication::{mouseButtons,keyboardModifiers}
to construct QDragMoveEvent and QDropEvent events. Which
results in the second reason why this is bad.

2) Platform plugins should not directly access member
variables of QGuiApplication, just to make sure that
QDragMoveEvent and QDropEvent events contain correct state.
Platform plugins should instead use QWindowSystemInterface
to communicate with Qt Gui (which is also the solution here).

The solution is to extend QWindowSystemInterface::handle{Drag,Drop}
to require mouse/keyboard state. We already do this for
some of the other methods, so it is nothing extraordinary.
This type of interface is also _required_ to support
drag-n-drops from other processes. We can't use
QGuiApplication::{mouseButtons,keyboardModifiers} when the
drag originates from another process, instead we need to
query mouse/keyboard state from the system.

This patch fixes drag-n-drops from others processes on XCB
platform plugin.

Task-number: QTBUG-57168
Change-Id: I3f8b0d2f76e9a32ae157622fef801829d629921d
Reviewed-by: Mikhail Svetkin <mikhail.svetkin@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

1 files changed, 6 insertions, 0 deletions

diff --git a/src/plugins/platforms/xcb/qxcbcursor.cpp b/src/plugins/platforms/xcb/qxcbcursor.cpp
index 8d151b760b..6795cb8905 100644
--- a/src/plugins/platforms/xcb/qxcbcursor.cpp
+++ b/src/plugins/platforms/xcb/qxcbcursor.cpp
@@ -626,6 +626,12 @@ xcb_cursor_t QXcbCursor::createBitmapCursor(QCursor *cursor)
 }
 #endif
 
+/*! \internal
+
+    Note that the logical state of a device (as seen by means of the protocol) may
+    lag the physical state if device event processing is frozen. See QueryPointer
+    in X11 protocol specification.
+*/
 void QXcbCursor::queryPointer(QXcbConnection *c, QXcbVirtualDesktop **virtualDesktop, QPoint *pos, int *keybMask)
 {
     if (pos)