scroll: Set tabIndex to -1 for simplebar content wrapper.

[shubham-padia]

Fixes #30403.

Having tabIndex set to 0 led to keyboard focus being put on a scrollbar container, which led to users having to tab twice to skip a container. This commit also removes instances of tabIndex being set to -1 programatically for certain cases, since it is -1 by default now. This commit also removes outline: none for simplebar since that property is not needed anymore becuase the wrapper is not focusable anymore.

The option became configurable in Grsmto/simplebar#695

The second commit is technically large, but I think all those changes belong together. Removing the previous hacks and css along with adding the property in the same commit made more sense to me. Reviewing it should be manage-able considering most of it just adding the tab-index property. Let me know if you think it should be split up

Screenshots and screen captures:

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: web/src/scroll_util.ts

Function	Unhandled Issue
get_scroll_element	TypeError: Cannot read properties of undefined (reading 'dataset') get_scroll_elemen... Event Count: 2 Affected Users: 1

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[timabbott]

@andersk can you review this one?

[timabbott]

Merged, thanks @shubham-padia and @andersk!