Press F12 to bring up the developer view, then centred at the top of the screen are the view options. At the top left is your pick-list of phones, and you can create custom ones if you don't see yours there. The circular icon at the extreme right of that horizontal menu is the landcape/portrait toggle.
Vertically along the RHS is the code and the log of interactions between client and server. In the top section if you select Elements (which is currently my default - it may be yours too) in the menu at the top and click through the HTML, as you highlight each element it adds colour to reveal where that is in the rendered page.
The entire left view (that of your 'mobile') is clickable/active.
It was this process that led me to the h3 for the offending popup options menu. It took some grepping to find where "when done click this header" lived in the code, bouncing me through the languages files before directing me back to header.php and my current solution.
- G.