Navbar toggle button opens then immediately closes

Question

I have a site using Bootstrap.

When the window size reduces to mobile, the Navbar menu correctly collapses to the usual hamburger (3-stripe) button.

My problem is that when I click this button to open the menu, it opens, but then immediately collapses again.

Answer 1

I was using a bootstrap 2.x CSS file, along with a bootstrap 3.x js file.