Inserter: search field autoFocus breaks withFocusReturn

[afercia]

The inserter was recently modified to have the search field as first thing in the inserter content and set initial focus on the search field using autoFocus.

Turns out this breaks the way withFocusReturn works. To reproduce:

• open the inserter using the keyboard
• observe the search field gets auto-focused
• tab inside the inserter menu
• press Escape
• focus is not moved back to the inserter toggle button

Setting focus on a component when it gets mounted is not compatible with how withFocusReturn gets the document.activeElement on componentDidMount. In this case, when the inserter menu gets mounted, the document.activeElement is the search field. Tested in Chrome and Firefox on macOS, this.activeElement in withFocusReturn actually returns the search field.

Question: why getting the active element on componentDidMount? Using componentWillMount would probably solve the issue. /cc @aduth