The decried behavior is not reproducible locally. You can also test this online demo
and verify that that the Insert Table dropdown tool is rendering as expected.
I suggest trying to isolate this layout issue into a separate clean project and provide it as attachment, so that it could be properly investigated.
Most probably this is a result from external CSS rules affecting the DIV elements globally in the page. I suggest trying to examine the set CSS stylization and see if any affects the dropdown element via the browser dev toolbar.
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