Media viewer controls rendered behind DSpace header

[ybnd]

Describe the bug
The preview of ngx-gallery is rendered behind the DSpace header, which obscures its controls, including the "Close" button. Because of this, the preview can only be closed by

• pressing Esc (which is likely not obvious to users)
• refreshing the page
• navigating to another page

To Reproduce
Steps to reproduce the behavior:

1. Open (or create) an Item with multiple images in the ORIGINAL Bundle
2. Click on the media viewer placeholder to open th preview

Expected behavior
The preview should be drawn over top of the header.

Solutions
Can be solved here by either

• not specifying a z-index (note that the z-index of ngx-gallery is already higher than the one we set here!)
• or specifying position: static

However, these changes are incompatible with mobile view (see #2220)

[HamzaFarooqArif]

I want to work on it. please assign it to me

[tdonohue]

Thanks again @HamzaFarooqArif for volunteering!

[HamzaFarooqArif]

I apologize that I'll not be able to continue working on it. I'm unassigning me from this issue

[tdonohue]

Additional details from duplicate ticket #2429 created by @Sondissimo :

Describe the bug
When we select an item, if we open the bitstream to visualize it, we encounter a strange behaviour regarding the top navbar.
Version: 7.6

To Reproduce
Steps to reproduce the behavior:

1. When we visualize an item bitstream the navbar is not correctly rendering
   

Expected behavior
The navbar shouldnt be visible when inspecting bitstreams

Related work
DSpace/DSpace#9022

[tdonohue]

Assigning to @Sondissimo who volunteered on behalf of 4Science.

[davide-negretti]

I'm now working on this one in place of @Sondissimo

[davide-negretti]

I opened a PR: #2594