QNX: Support window translucency

A few tactical changes were necessary to do this:

* The root window's buffer must be the full size of the window in
  order for transparency to work. We now transition this buffer
  to full-size upon first request of some QWindow to be translucent.

  At the same time the root window is transitioned to being full-
  screen, we also set its alpha blending mode to Source-Over in
  order to permit lower z-ordered apps to show through.

* Set the root window's buffer position to (0, 0) to avoid a momentary
  flicker of unpainted pixels on the far right-hand border as a
  translucent window comes on-screen.

* Use Source-Over alpha blending for normal child windows too.

The QtGui core logic for setting the backing surface pixel format to
something with an alpha channel if Qt::WA_TranslucentBackground is
set on the toplevel QWidget already accomplished the remainder of
the work.

Change-Id: I1c2b31aa6323b4555fa194313f0f5e06ada494fc
Reviewed-by: Kevin Krammer <kevin.krammer@kdab.com>
Reviewed-by: Thomas McGuire <thomas.mcguire@kdab.com>
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>
Reviewed-by: Andreas Holzammer <andreas.holzammer@kdab.com>

1 files changed, 12 insertions, 2 deletions

diff --git a/src/plugins/platforms/qnx/qqnxwindow.cpp b/src/plugins/platforms/qnx/qqnxwindow.cpp
index f4cbb0e9db..7a1ffb98b8 100644
--- a/src/plugins/platforms/qnx/qqnxwindow.cpp
+++ b/src/plugins/platforms/qnx/qqnxwindow.cpp
@@ -117,9 +117,13 @@ QQnxWindow::QQnxWindow(QWindow *window, screen_context_t context)
     if (result != 0)
         qFatal("QQnxWindow: failed to set window alpha mode, errno=%d", errno);
 
-    // Make the window opaque
+    // Blend the window with Source Over Porter-Duff behavior onto whatever's
+    // behind it.
+    //
+    // If the desired use-case is opaque, the Widget painting framework will
+    // already fill in the alpha channel with full opacity.
     errno = 0;
-    val = SCREEN_TRANSPARENCY_NONE;
+    val = SCREEN_TRANSPARENCY_SOURCE_OVER;
     result = screen_set_window_property_iv(m_window, SCREEN_PROPERTY_TRANSPARENCY, &val);
     if (result != 0)
         qFatal("QQnxWindow: failed to set window transparency, errno=%d", errno);
@@ -370,6 +374,12 @@ void QQnxWindow::setBufferSize(const QSize &size)
             qWarning() << "QQnxWindow: Buffer size was" << size;
             qFatal("QQnxWindow: failed to create window buffers, errno=%d", errno);
 
+        // If the child window has been configured for transparency, lazily create
+        // a full-screen buffer to back the root window.
+        if (window()->requestedFormat().hasAlpha()) {
+            m_screen->rootWindow()->makeTranslucent();
+        }
+
         // check if there are any buffers available
         int bufferCount = 0;
         result = screen_get_window_property_iv(m_window, SCREEN_PROPERTY_RENDER_BUFFER_COUNT, &bufferCount);