tdf#124579: ensure to provide an event to wake up main loop when notifying

Without that, Request::waitProcessMessages might wait indefinitely for
Application::Yield() to return; while the latter would wait for a message
in GetMessage. If there's no visible LO window, the message might never
arrive by itself.
Co-authored-by: Jan-Marek Glogowski <glogow@fbihome.de>

Change-Id: Ie2622053a8d4467eb1cbd579d8496cb5ddef08aa
Reviewed-on: https://gerrit.libreoffice.org/70346Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>
Tested-by: Mike Kaganski <mike.kaganski@collabora.com>

fpicker/source/win32/asyncrequests.cxx

@@ -19,6 +19,7 @@
 
 #include "asyncrequests.hxx"
 #include <vcl/svapp.hxx>
+#include <vcl/winscheduler.hxx>
 #include <osl/mutex.hxx>
 
 namespace fpicker{
@@ -56,6 +57,10 @@ void Request::waitProcessMessages()
 void Request::notify()
 {
     m_aJoiner.set();
+    // Make sure that main loop receives at least this message to return from GetMessage and recheck
+    // the condition, even in case when there's no visible application windows present, and thus no
+    // other messages might arrive to the main loop.
+    WinScheduler::PostDummyMessage();
 }
 
 AsyncRequests::AsyncRequests(const RequestHandlerRef& rHandler)

include/vcl/winscheduler.hxx

@@ -31,6 +31,11 @@ struct VCL_DLLPUBLIC WinScheduler final
     /// Hack for Windows native dialogs, which run the main loop, so we can't
     /// use the direct processing shortcut.
     static void SetForceRealTimer();
+
+    // Posting a dummy messages might be needed to ensure that main loop eventually returns from
+    // GetMessage ("wakes up") in modes without a visible window. This, e.g., might be needed to
+    // let main thread recheck a condition set from another thread.
+    static void PostDummyMessage();
 };
 
 #endif // _WIN32

vcl/source/app/winscheduler.cxx

@@ -25,12 +25,18 @@
 #include <win/saldata.hxx>
 #include <win/salinst.h>
 
-void WinScheduler::SetForceRealTimer()
+namespace
 {
-    BOOL const ret
-        = PostMessageW(GetSalData()->mpInstance->mhComWnd, SAL_MSG_FORCE_REAL_TIMER, 0, 0);
+void PostMessageToComWnd(UINT nMsg)
+{
+    BOOL const ret = PostMessageW(GetSalData()->mpInstance->mhComWnd, nMsg, 0, 0);
     SAL_WARN_IF(0 == ret, "vcl.schedule", "ERROR: PostMessage() failed!");
 }
+}
+
+void WinScheduler::SetForceRealTimer() { PostMessageToComWnd(SAL_MSG_FORCE_REAL_TIMER); }
+
+void WinScheduler::PostDummyMessage() { PostMessageToComWnd(SAL_MSG_DUMMY); }
 
 #endif // _WIN32