Fix read-only text browser sometimes jump to the top of the document

Respect the inputMethodAccepted() attribute of QPlatformInputContext in
the ibus input context plugin. This is what for example the windows
input context plugin does, too.

A text browser that is read-only should not receive input method events
that include a cursor placement. So when it receives the focus, we must
only convey the received-focus information to the input method if the
widget is editable (or accepts input method events generally speaking),
because as soon as the ibus process learns about it, it will send an
input method event that looks like a "let us reset the state a clean
start and place the cursor at the beginning" message. We are not
interested in reaching that state with the ibus process.

Task-number: QTBUG-63066
Change-Id: I1b0e5f8a396bc31169d6081f9325092b447cf60a
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforminputcontexts/ibus/qibusplatforminputcontext.cpp

@@ -254,6 +254,12 @@ void QIBusPlatformInputContext::setFocusObject(QObject *object)
     if (!d->busConnected)
         return;
 
+    // It would seem natural here to call FocusOut() on the input method if we
+    // transition from an IME accepted focus object to one that does not accept it.
+    // Mysteriously however that is not sufficient to fix bug QTBUG-63066.
+    if (!inputMethodAccepted())
+        return;
+
     if (debug)
         qDebug() << "setFocusObject" << object;
     if (object)