WebToolbarMenu wrong position

I have two WebToolbars on the same page, one right under the other. In the lower Toolbar I have a WebToolbarMenu. This all displays fine, except when I click on the menu, the popup menu displays in the wrong location, ABOVE its toolbar.

It’s as if the vertical position of the menu is hardwired to be a constant relative to the top of the page rather than the top of the WebToolbar.

Is this a bug or am I doing something wrong?

Xojo 2018r3, macOS 10.13.6. Same behavior in Safari, Chrome, and Firefox.