After migrating from TT6 to TT7 I observed that the Hamburger Menu in the mobile view does not show up in the code generated by TT7. This worked in TT6. A hint may be the following code difference in page.tpl.php. Adding the <span> directives fixed the problem. Doing it manually is not the perfect solution.
TT6:
<div id="navigationmenu">
<div class="navbar-header">
<button id="nav-expander" data-target=".navbar-collapse" data-toggle="collapse" class="navbar-toggle" type="button">
<span class="sr-only">
</span>
<span class="icon-bar">
</span>
<span class="icon-bar">
</span>
<span class="icon-bar">
</span>
</button>
</div>
TT7:
<div id="navigationmenu">
<div class="navbar-header">
<button id="nav-expander" data-target=".navbar-collapse" data-toggle="collapse" class="navbar-toggle" type="button">
</button>
</div>