fix: Apply stacked flashbar feature shadows to correct element

[jkuelz]

Description

The recent shadow updates in #702, unified how we apply shadows in many components, including simplifying the shadow approach in Flashbars. Previously, Flashbars applied shadows on different elements depending on the theme (pseudo element in refresh, directly on the Flash item in classic). However the latest update simplified both themes to use a single approach (pseudo element) (see code diff).

This unintentionally brought a tiny existing bug to light in the unreleased stacked Flashbar feature. More specifically, stacked Flashbars should have a different shadow compared to a normal un-stacked Flashbar shadow. This was the case in classic but not refresh - because shadows previously were applied to separate elements, the shadow was overriding the original shadows in classic (intended), but it was adding them to the default Flashbar shadow in visual refresh resulting in a double shadow (not intended).

This bug was barely noticeable to the human eye due to the large spread of shadows in visual refresh, which is why it did not get caught until now. The refactoring in #702, brought this the same bug that was happening in visual refresh to appear in classic as well, resulting in a double shadow on all stacked Flashbars (in classic and refresh - previously just refresh), and is more noticeable in classic as seen in the screenshots below (albeit still very subtle).

This change also addresses a bug in dark mode visual refresh shadow that was causing it to overlap underlying Flashbars, making them almost invisible.

Screenshots below, each highlighting:

before #702 | after #702 | after #702 with this fix

Related links, issue #, if available: n/a

How has this been tested?

Locally, part of the reason this regression happened in the first place was because there were no tests in place to catch it. These are coming in a separate PR as part of the stacked Flashbar feature release.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

Base: 93.27% // Head: 93.27% // No change to project coverage 👍

Coverage data is based on head (bc8f68c) compared to base (76e0135).
Patch has no changes to coverable lines.

Additional details and impacted files

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  Coverage   93.27%   93.27%           
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  Files         584      584           
  Lines       17074    17074           
  Branches     4703     4703           
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  Hits        15926    15926           
  Misses       1068     1068           
  Partials       80       80           

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