Revert: Fix/editor content zindex stack context

[ironprogrammer]

What?

Reverts #38893, "Raise z-index of content div relative to sidebars" (commit b90f46f). This change was introduced to allow the editor's main content area to display link popover modals above the sidebars.

Also reverts a subsequent corrective change in #39331, which was opened due to the original content area z-index change (commit 2a18443).

Why?

While #38893 addressed the original issue (#38723), it introduced other downstream issues that rely on the content area appearing at a lower z-axis than the sidebars.

These issues include:

• Templates page: reduce z-index of content area #39331
• Inserter: Preview panel not showing up #39582
• https://github.com/Automattic/p2/pull/4930

Because of the outsized impact of addressing the original minor issue, there was consensus that the z-axis change should be reverted, and addressed differently. Here are the requests, for reference:

• Inserter: Preview panel not showing up #39582 (comment)
• Raise z-index of content div relative to sidebars #38893 (comment)

How?

Reversion of the change restores .interface-interface-skeleton__content to z-index: 20. Also rolls back a separate and subsequent adjustment applied for visibility of the Templates "Add New" dropdown menu in #39331.

Testing Instructions

Check out PR or apply the patch to gutenberg:trunk and re-build the plugin. Make sure the plugin is activated.

Test 1: Original Issue #38723 (un-fix)

1. On medium and up screen (tablet, laptop, desktop):
2. Create new or edit a post.
3. Open the List View and/or Settings sidebars.
4. Enter test paragraph content, if needed, and highlight some text -- text on the extreme left or right of the paragraph works best to replicate the issue.
5. Click the "link" option from the inline toolbar, and the Link popover (modal) will activate.
6. Observe that the popover appears "under" either sidebar (⚠️ this un-fixes the original issue, Block Toolbar: Link Tooltip overlapping #38723).

Screenshots

Before Patch

Original fix shows link popover "above" sidebars.

After Patch

Link popover appears "under" sidebars, restoring original behavior.

Regression for #32869

Actions panel retains stacking priority, and popover appears "under" sidebars.

Test 2: Templates "Add New" Dropdown Visibility #39331 (revert)

1. With a block theme activated, navigate to Appearance > Editor (/wp-admin/site-editor.php).
2. Click the WordPress icon in the upper-left corner to toggle navigation.
3. Click Templates to open the template selector.
4. Click the "Add New" button to activate the "Add New Template" dropdown menu.
5. Observe that the dropdown is visible and appears "above" the template list.

Screenshots

Regression for #39331

"Add New" dropdown menu remains visible.

[ramonjd]

Thanks for the revert PR!

I guess #39331 will have to be reverted along with these changes as well? Would it be possible to bundle that into this PR for ease of testing?

[ironprogrammer]

I guess #39331 will have to be reverted along with these changes as well? Would it be possible to bundle that into this PR for ease of testing?

Hi, @ramonjd -- absolutely. I'll slurp in those revert changes as a separate commit.

[ramonjd]

Looks good to me.

Original behaviour is restored for the link and template popovers.

[mtias]

Thanks!