fix(dropdown): focus-out handling when new focus comes from another dropdown-toggle (closes #4113)

[jacobmllr95]

Describe the PR

This PR fixes the <b-dropdown> and <b-nav-item-dropdown> (when vertical and inline menu is displayed) close handling when focus is lost by clicking an item below, and the item's click handler not firing due to the position of the item moving before the click handler gets a chance to be triggered.

Uses a setTimeout delay to postpone the drop down menu collapse (when in a navbar) when the menu looses focus.

Closes #4113.

PR checklist

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Enhancement
• ARIA accessibility
• Documentation update
• Other (please describe)

Does this PR introduce a breaking change? (check one)

• No
• Yes (please describe)

The PR fulfills these requirements:

• It's submitted to the dev branch, not the master branch
• When resolving a specific issue, it's referenced in the PR's title (i.e. [...] (fixes #xxx[,#xxx]), where "xxx" is the issue number)
• It should address only one issue or feature. If adding multiple features or fixing a bug and adding a new feature, break them into separate PRs if at all possible.
• The title should follow the Conventional Commits naming convention (i.e. fix(alert): not alerting during SSR render, docs(badge): update pill examples, fix typos, chore: fix typo in README, etc). This is very important, as the CHANGELOG is generated from these messages.

If new features/enhancement/fixes are added or changed:

• Includes documentation updates (including updating the component's package.json for slot and event changes)
• Includes any needed TypeScript declaration file updates
• New/updated tests are included and passing (if required)
• Existing test suites are passing
• The changes have not impacted the functionality of other components or directives
• ARIA Accessibility has been taken into consideration (Does it affect screen reader users or keyboard only users? Clickable items should be in the tab index, etc.)

If adding a new feature, or changing the functionality of an existing feature, the PR's
description above includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

[codecov]

Codecov Report

Merging #4139 into dev will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev    #4139      +/-   ##
==========================================
+ Coverage   99.86%   99.86%   +<.01%     
==========================================
  Files         238      238              
  Lines        4471     4473       +2     
  Branches     1260     1261       +1     
==========================================
+ Hits         4465     4467       +2     
  Misses          5        5              
  Partials        1        1

Impacted Files	Coverage Δ
src/utils/dom.js	100% <100%> (ø)	⬆️
src/mixins/dropdown.js	100% <100%> (ø)	⬆️

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[tmorehouse]

Playground test markup:

<div>
  <b-navbar type="dark" variant="dark" toggleable>
    <b-navbar-nav>
      <b-nav-item href="#">Home</b-nav-item>
      <!-- Navbar dropdowns -->
      <b-nav-item-dropdown text="Lang" right id="lang-dd">
        <b-dropdown-item href="#">EN</b-dropdown-item>
        <b-dropdown-item href="#">ES</b-dropdown-item>
        <b-dropdown-item href="#">RU</b-dropdown-item>
        <b-dropdown-item href="#">FA</b-dropdown-item>
      </b-nav-item-dropdown>
      <b-nav-item-dropdown text="User" right id="user-dd">
        <b-dropdown-item href="#">Account</b-dropdown-item>
        <b-dropdown-item href="#">Settings</b-dropdown-item>
      </b-nav-item-dropdown>
    </b-navbar-nav>
  </b-navbar>
</div>

[tmorehouse]

Hmmm... works better.. but back to the same issue with keyboard (TAB) navigation not closing the dropdown (if a dropdown toggle is next in the tab sequence)