Fix bug with "pass through" mouse events from MSW wxFileDialog

Remove any remaining mouse messages from the input queue after showing
the file dialog because they somehow remain after it is destroyed and
could be processed by the application once wxFileDialog::ShowModal()
returns, resulting in phantom events being processed by the controls
that just happen to be under the button of the file dialog that was
clicked to close it.

See wxWidgets#10924, wxWidgets#23944.

(cherry picked from commit ffc99bd)

Author: charvey2718

docs/changes.txt

@@ -327,6 +327,7 @@ wxMSW:
 - Fix background colour of empty cells in wxDataViewCtrl (#23708).
 - Fix possible (even if rare) crash in printer progress dialog (#23927).
 - Fix "+" button not appearing in wxTreeCtrl after adding a child (#23718).
+- Fix phantom mouse events after dismissing wxFileDialog (charvey2718, #10924).
 
 wxOSX:
 

src/msw/filedlg.cpp

@@ -113,6 +113,33 @@ bool gs_changedPolicy = false;
 
 #endif // #if wxUSE_DYNLIB_CLASS
 
+/*
+    This function removes any remaining mouse messages from the input queue in
+    order to prevent them from being passed to controls positioned underneath
+    the wxFileDialog after it has been destroyed, see #10924.
+*/
+void DrainMouseMessages()
+{
+    // Note that we have to use this struct as PeekMessage() wouldn't remove
+    // the messages from the input queue, even with PM_REMOVE, if we pass it a
+    // null pointer.
+    MSG msg;
+
+    // This loop is used just to ensure that we don't loop indefinitely in case
+    // there is something generating an endless stream of mouse messages in the
+    // system (1000 is an arbitrary but "sufficiently large" number), the real
+    // loop termination condition is inside it.
+    for ( int i = 0; i < 1000; ++i )
+    {
+        if ( !::PeekMessage(&msg, NULL, WM_MOUSEFIRST, WM_MOUSELAST,
+                            PM_REMOVE | PM_QS_INPUT) )
+        {
+            // No more mouse messages left.
+            break;
+        }
+    }
+}
+
 /*
 Since Windows 7 by default (callback) exceptions aren't swallowed anymore
 with native x64 applications. Exceptions can occur in a file dialog when
@@ -1493,6 +1520,8 @@ int wxFileDialog::ShowCommFileDialog(WXHWND hWndParent)
     DWORD errCode;
     bool success = DoShowCommFileDialog(&of, m_windowStyle, &errCode);
 
+    DrainMouseMessages();
+
     // When using a hook, our HWND was set from MSWOnInitDialogHook() called
     // above, but it's not valid any longer once the dialog was destroyed, so
     // reset it now.
@@ -1696,6 +1725,9 @@ int wxFileDialog::ShowIFileDialog(WXHWND hWndParent)
 
     // Finally do show the dialog.
     const int rc = fileDialog.Show(hWndParent, options, &m_fileNames, &m_path);
+
+    DrainMouseMessages();
+
     if ( rc == wxID_OK )
     {
         // As with the common dialog, the index is 1-based here, but don't make