Prevent sending EVT_MOUSE_CAPTURE_LOST while opening a context menu in wxSTC

At least under wxMSW and wxGTK opening a popup menu when mouse is captured
generates EVT_MOUSE_CAPTURE_LOST. We would like to avoid this and
to release capture in a controlled way.

See #17961.

src/stc/ScintillaWX.cpp

@@ -1165,6 +1165,10 @@ void ScintillaWX::DoCommand(int ID) {
 bool ScintillaWX::DoContextMenu(Point pt) {
     if (ShouldDisplayPopup(pt))
     {
+        // To prevent generating EVT_MOUSE_CAPTURE_LOST.
+        if ( HaveMouseCapture() ) {
+            SetMouseCapture(false);
+        }
         ContextMenu(pt);
         return true;
     }