Overlay cannot focus autofocus elements due to React behavior

[michael-yx-wu]

Environment

• Package version(s): @blueprintjs/[email protected]
• Operating System: any
• Browser name and version: any

Filling as a follow up to #4894 (comment)

When setting the autoFocus prop on a component, React chooses not to render it to the DOM (see facebook/react#11851 (comment)), so this selector will never find anything:

blueprint/packages/core/src/components/overlay/overlay.tsx

Line 351 in 2c71d6a

const autofocusElement = this.containerElement.querySelector("[autofocus]") as HTMLElement;

On initial open of the overlay, the autofocus element should get focus thanks to React calling Element#focus under the hood, but subsequent runs of this function will not focus the autofocus element. When returning focus back to the Overlay, do we always want to focus the first autofocus element? If so, we may need to consider using cloning children and replacing autoFocus with autofocus (no caps), but this will probably result console warnings. If not, we can just delete this line and rely on React.