Revert "A11Y: Do not use positive tabindex in composer (#12563)" (PR #12603)

This reverts commit af879b7851e3c26b9a612dc7d76ca02c80a619c6.

The reverted commit made keyboard focusable content in the preview interrupt tab order, we’ll need to do something more complex here… like adding tabindex="-1" to all preview content.

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