I have made a test for the container property of TWebMainMenu component:
I put a Webpanel1 aligned at Top, and set webpanel1 as the Twebmainmenu's container and it works. And displaying of mainmenu is fine(see below picture).
However, I then out an another webpanel2 and set Webpanel1 as its parent, aligning to right of the webpanel1. There will be issue of displaying the mainmenu, as you can see from below screenshot:
Lower part of the mainmenu was covered, thus menu cannot be displayed completely.
For your reference, here is the test file : 61. mainmenu.zip (1.4 MB)