I actually solved this while getting a screenshot for this case - but thought I'd post in case anyone else comes across it.
My setup is a Horizontal Sidebar, with Map and WidgetController (Bookmarks,Filters) in the First panel. A List in the second panel. I was focussing on the mobile layout but found about a third of the map was not responsive to touch navigation. The touch deadzone is the same area that the widget panels open up in.
But when looking at the page outline, you can see that my WidgetController sits underneath the Map component. Moving the WidgetController so that it, and the map both sit directly under the 'First' panel, solved the issue. (Move to Pending is your friend when trying to move components)
Solved! Go to Solution.