I installed Odoo15 two weeks ago and I found it had different behavior in navbar expansion than that of Odoo 14. Usually, one would expect the navigation bar of the menu horizontally displayed like menu1 menu2 menu3 ... until the right end of screen where the remaining menu, if any, will be display as a "+" sign, clicking which you will see the remaining menu. In RWD, if the screen is small then you will not see menu expanded, instead, you will see a hamburg icon (three short rows)  clicking which, you will see all the menu displayed vertically, from which you can click the one you want. It is the way most website do and so is odoo14.  But in Odoo 15, no matter what the screen size is, you all see an Hamburg icon. I don't know if this is the design philosophy adopted by Odoo15 or it is an accidental mistake. I find no way to select otherwise. I check the website_template.xml of Odoo15 against that of Odoo14, and find that the problem is in a statement: 

I use goggle inspect to see the code rendered is: 

the key component to let navbar expand is "navbar-expand-lg", which appears in Odoo14 but not in Odoo15, so I added to it with google web tool,like below

Immediately, the navigation bar expansion works again as in Odoo14.

I add it to website_template.xml as 

It works! 

However, strange things happen, several days later, I find my website behavior goes back to the way I change it before, and I check the code, it is still there, I repeat the modification on website with Google web tool, it works again, but changing the setting to website_template.xml cannot generate the essential 'navbar-expand-lg' again. 

Did any one try Odoo15 and see this phenomena? Advice is urgently needed since I am working on Odoo15 to do a project. Your help is highly appreciated.