Don't generate bogus root item selection events in wxMSW wxTreeCtrl.

Setting focus to the control when receiving a selection event can result in
another selection event being generated by the control itself if it hadn't had
any selection before, which is completely artificial, i.e. doesn't correspond
to any user action, and so has to be suppressed.

Closes #16999.

src/msw/treectrl.cpp

@@ -3477,7 +3477,14 @@ bool wxTreeCtrl::MSWOnNotify(int idCtrl, WXLPARAM lParam, WXLPARAM *result)
             // to avoid such surprises, we force the generation of focus events
             // now, before we generate the selection change ones
             if ( !m_changingSelection && !m_isBeingDeleted )
+            {
+                // Setting focus can generate selection events too however,
+                // suppress them as they're completely artificial and we'll
+                // generate the real ones soon.
+                TempSetter set(m_changingSelection);
+
                 SetFocus();
+            }
             break;
 
         // instead of explicitly checking for _WIN32_IE, check if the