fix(dropdowns): auto-focus only once dropdown is properly positioned

What does this MR do?

fix(dropdowns): auto-focus only once dropdown is properly positioned

This ensures that we wait on dropdowns to be properly positioned within the page before attempting to auto-focus some of its child elements. Without this, the auto-focus might cause the page to unexpectedly scroll back to the top.

There is a simpler approach

When I finished implementing this fix, it occurred to me that a much simpler approach might work:

diff --git a/src/components/base/new_dropdowns/base_dropdown/base_dropdown.vue b/src/components/base/new_dropdowns/base_dropdown/base_dropdown.vue
index 8688008f..2936d028 100644
--- a/src/components/base/new_dropdowns/base_dropdown/base_dropdown.vue
+++ b/src/components/base/new_dropdowns/base_dropdown/base_dropdown.vue
@@ -222,8 +222,8 @@ export default {
       return {
         'gl-display-block!': this.visible,
         [FIXED_WIDTH_CLASS]: !this.fluidWidth,
-        'gl-fixed': this.isFixed,
-        'gl-absolute': !this.isFixed,
+        'gl-fixed': this.openedYet && this.isFixed,
+        'gl-absolute': this.openedYet && !this.isFixed,
       };
     },
     isFixed() {
@@ -324,6 +324,7 @@ export default {
          * dropdown actually being visible.
          */
         await this.$nextTick();
+        this.openedYet = true;
         this.startFloating();
         this.$emit(GL_DROPDOWN_SHOWN);
       } else {

And indeed it works. Delaying the application of the position CSS property seems to ensure that the auto-focus always happens when the dropdown is close to the toggle. I did not push that approach simply because I'm not sure I fully understand the underlying timing. E.g. how are the dropdown's coordinates properly computed if it's not absolutely positioned yet? I'm concerned that the fact that this works is a mere coincidence and can't be relied on. I figured I would rather go with a slightly more complex solution that I can fully grasp.

Screenshots or screen recordings

Before	After
scroll	no_scroll

Integration merge requests

Creating an integration MR might be messy currently as @gitlab/ui is way outdated in GitLab. The changes can be verified by:

1. Pulling gitlab!121070 (merged) locally and running yarn. At this point, you should be on a version that has the scrolling issue. This can be verified in http://gdk.test:3000/-/profile/notifications.
2. Installing the dev build in the GDK:

   yarn add @gitlab/ui@https://gitlab.com/gitlab-org/gitlab-ui/-/jobs/4379481074/artifacts/raw/gitlab-ui.dropdowns-await-compute-position.tgz

   You're now on this fixed version, the page should not scroll unexpectedly anymore.

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