Fix CSS so TinyMCE sslink popup windows show over fullscreen

[lianna-blanca]

[WIP WIP WIP]

Description

When using TinyMCE in CMS 5.2, the editor's fullscreen plugin hides the pop-up windows which allow users to configure links. This applies to all types of links (internal, external, file, anchor, email, phone).

This appears to be due to the TinyMCE fullscreen plugin having a z-index of 1200, whereas our popup windows are at 1050. When I manually change this in chrome dev tools, the popups appear over the full screen as they should.

A very similar problem was brought up as an issue on the TinyMCE repo in 2021, but the issue was closed as intended behaviour. (There also appears to be a number of other related issues, but none that look like they'll be remedied in the near future.) Therefore we will need to address the issue here.

Steps to reproduce

Steps for reproducing in project skeleton, or a similar project:

1. Go to any TinyMCE editor field which has both sslink (which we have by default in this repo and the fullscreen plugin:

// Project Skeleton file: `app/_config.php`
// Add fullscreen plugin:
$cmsEditor = TinyMCEConfig::get('cms');
$cmsEditor->enablePlugins('fullscreen');
$cmsEditor->addButtonsToLine(2, 'fullscreen');

2. Open full screen mode
   
3. Press the sslink button, and select any option from the dropdown field
   
4. Observe that no popup appears
5. Close full screen mode; observe that the popup for editing that link type is now visible.
   

Manual testing steps

Issues

• TinyMCE sslink plugin popup windows are hidden in full screen mode #1815

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