[beta] rustdoc ui: adjust tooltip z-index to be above sidebar

[lukas-code]

Backport of #119477.
Fixes #119472.

range-diff:

446:  d796ad4209b =  11:  53637cdd8ed rustdoc ui: adjust tooltip z-index to be above sidebar
459:  b1853eb3bd5 !  12:  c4c4ff67fae use css variable for z-index of the sidebar
    @@ src/librustdoc/html/static/css/rustdoc.css: so that we can apply CSS-filters to
        margin-top: 7px;
        border-radius: 3px;
        border: 1px solid var(--border-color);
    -@@ src/librustdoc/html/static/css/rustdoc.css: a.tooltip:hover::after {
    - }
    - .src #sidebar-button {
    -   left: 8px;
    --  z-index: 101;
    -+  z-index: calc(var(--desktop-sidebar-z-index) + 1);
    - }
    - .hide-sidebar .src #sidebar-button {
    -   position: static;

The "show sidebar" button on the source view page works differently on beta and nightly, but it is in fact above the sidebar in both versions.

beta button:

nightly button:

cc @Mark-Simulacrum

[rustbot]

r? @notriddle

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[rustbot]

⚠️ Warning ⚠️

• Pull requests are usually filed against the master branch for this repo, but this one is against beta. Please double check that you specified the right target!

[rustbot]

Some changes occurred in GUI tests.

cc @GuillaumeGomez

Some changes occurred in HTML/CSS/JS.

cc @GuillaumeGomez, @jsha

[notriddle]

This seems fine to me. Should I r+ it, or should someone on the release team do that?

[Mark-Simulacrum]

@bors r=notriddle rollup=never

[bors]

📌 Commit c4c4ff6 has been approved by notriddle

It is now in the queue for this repository.

[bors]

⌛ Testing commit c4c4ff6 with merge 7bcd95c...

[bors]

☀️ Test successful - checks-actions
Approved by: notriddle
Pushing 7bcd95c to beta...