QtPlatformHeaders/Windows: Add function to set window activation behavior.

The Windows OS by default does not activate windows when the calling
process is not active; only the taskbar entry is flashed as not to
distract the user.
Nevertheless, for some use cases, it is desirable to activate the window
also in the inactive state.
Introduce an enumeration specifying the behavior to QtPlatformHeaders
and employ a workaround using the Win32 API AttachThreadInput() to attach
to other processes while setting the foreground window to achieve
the AlwaysActivateWindow behavior.

Task-number: QTBUG-14062
Task-number: QTBUG-37435
Change-Id: I79cb6cd3fab29d55b5d3db7f9af01bbaa5096a37
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

1 files changed, 23 insertions, 0 deletions

diff --git a/src/plugins/platforms/windows/qwindowswindow.cpp b/src/plugins/platforms/windows/qwindowswindow.cpp
index 4c503a2c48..c1a047e276 100644
--- a/src/plugins/platforms/windows/qwindowswindow.cpp
+++ b/src/plugins/platforms/windows/qwindowswindow.cpp
@@ -43,6 +43,7 @@
 #include "qwindowsdrag.h"
 #include "qwindowsscreen.h"
 #include "qwindowsintegration.h"
+#include "qwindowsnativeinterface.h"
 #include "qwindowsopenglcontext.h"
 #ifdef QT_NO_CURSOR
 #  include "qwindowscursor.h"
@@ -2083,8 +2084,30 @@ void QWindowsWindow::requestActivateWindow()
     // 'Active' state handling is based in focus since it needs to work for
     // child windows as well.
     if (m_data.hwnd) {
+#ifndef Q_OS_WINCE
+        const DWORD currentThread = GetCurrentThreadId();
+        bool attached = false;
+        DWORD foregroundThread = 0;
+
+        // QTBUG-14062, QTBUG-37435: Windows normally only flashes the taskbar entry
+        // when activating windows of inactive applications. Attach to the input of the
+        // currently active window while setting the foreground window to always activate
+        // the window when desired.
+        if (QGuiApplication::applicationState() != Qt::ApplicationActive
+            && QWindowsNativeInterface::windowActivationBehavior() == QWindowsWindowFunctions::AlwaysActivateWindow) {
+            if (const HWND foregroundWindow = GetForegroundWindow()) {
+                foregroundThread = GetWindowThreadProcessId(foregroundWindow, NULL);
+                if (foregroundThread && foregroundThread != currentThread)
+                    attached = AttachThreadInput(foregroundThread, currentThread, TRUE) == TRUE;
+            }
+        }
+#endif // !Q_OS_WINCE
         SetForegroundWindow(m_data.hwnd);
         SetFocus(m_data.hwnd);
+#ifndef Q_OS_WINCE
+        if (attached)
+            AttachThreadInput(foregroundThread, currentThread, FALSE);
+#endif // !Q_OS_WINCE
     }
 }