Following on from @nray's analysis in T260412#6419741 we will switch to a header-first DOM
We will tie the move to the existing $wgVectorIsSearchInHeader feature flag. This means that the rollout of the DOM changes will not impact cached HTML
# Acceptance criteria
[] When VectorIsSearchInHeader is enabled, a header-first DOM will be used
[] When VectorIsSearchInHeader is disabled, the existing DOM will be used
[] All CSS required by the existing layout that is not needed by the new layout will be tied to the body class `skin-vector-search-header-legacy` which is currently used on all pages.
[] All CSS required by the new DOM based layout that is not needed by the new layout will be tied to the new body class `skin-vector-search-header` which will only appear when we enable the feature flag
[] On first deploy, `VectorIsSearchInHeader` feature flag should be disabled so that the new CSS gets cached before DOM changes. When new CSS is cached, the feature flag can then be enabled.
# QA
[] A developer should verify that the HTML layout when $wgVectorIsSearchInHeader = false is not changed by the patch that introduces the header-first DOM
# Sign off
[] With this change, the sidebar is planned to be moved out of the header and into the `.mw-workspace-container`. One negative side effect of this change is that users will no longer be able to tab from the sidebar button into the sidebar without us using JavaScript. Create a task for implementing the ability to tab from the sidebar button into the sidebar