Thank you for the provided image as now we can better see the reason of the issue.
The main functionality of splitters is co cut all overflowing parts of its children. As the menu is a children of a splitter, all overflowing parts of the menu (in this case a popup) are cut. There is no workaround to fix the issue, as CSS do not provide solution to set which children will be cut and which not, as overflowing property can be set only to parents elements.
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