xcb: reduce focus-in delay

This patch amends fe63900dc9

The previous value of 400ms was a random and seemingly harmless choice.
It turns out that 400ms is a bit too long and interferes with x11 async
behavior tweaks in the VirtualBox source code.

The original aim of specifying a concrete delay was to fix the
nondeterministic behavior of the pre-existing code and to avoid flickering
on KWin caused by waiting too little. This patch changes 400ms -> 100ms,
which seems to work better in practice.

Fixes: QTBUG-76742
Change-Id: Ia8168216819ac41d0124622c9472a98a1877262f
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/plugins/platforms/xcb/qxcbconnection.cpp

@@ -132,7 +132,9 @@ QXcbConnection::QXcbConnection(QXcbNativeInterface *nativeInterface, bool canGra
     if (!m_startupId.isNull())
         qunsetenv("DESKTOP_STARTUP_ID");
 
+    const int focusInDelay = 100;
     m_focusInTimer.setSingleShot(true);
+    m_focusInTimer.setInterval(focusInDelay);
     m_focusInTimer.callOnTimeout([]() {
         // No FocusIn events for us, proceed with FocusOut normally.
         QWindowSystemInterface::handleWindowActivated(nullptr, Qt::ActiveWindowFocusReason);

src/plugins/platforms/xcb/qxcbwindow.cpp

@@ -848,7 +848,7 @@ void QXcbWindow::doFocusOut()
     connection()->setFocusWindow(nullptr);
     relayFocusToModalWindow();
     // Do not set the active window to nullptr if there is a FocusIn coming.
-    connection()->focusInTimer().start(400);
+    connection()->focusInTimer().start();
 }
 
 struct QtMotifWmHints {