In the core theme & in sub-themes derived from it can happen that the hamburger icon & the ability to slide out the navigation sidebar disappears.
Clean install of theme or after running script to install sub-theme. Remove all blocks in the region navigation, leaving the block Main Navigation in the region navigation_collapsible. After cache clear & refresh the hamburger icon is no longer rendered.
I believe when a region is empty of blocks the attached libraries are not loaded. Thus, in the [dot]theme file for the root theme or sub-theme, in the function that Implements template_preprocess_region() the switch case is attaching the library dxpr-theme-header--mobile to the region navigation & not to the region navigation_collapsible.
So adding to the code the line below my comment:
case 'navigation':
// GM Added here because if above region has no content hamburger vanishes.
case 'navigation_collapsible':
$variables['#attached']['library'][] = 'entrebahn_dxpr_subtheme/dxpr-theme-header';
$variables['#attached']['library'][] = 'entrebahn_dxpr_subtheme/dxpr-theme-header--top';
$variables['#attached']['library'][] = 'entrebahn_dxpr_subtheme/dxpr-theme-header--mobile';
break;
Fixes the issue & allows either region to be empty of blocks.
Fixed
5.1
Code