[iOS] Inline editor does not stick when scrolled off screen

[kinghenry]

Steps to reproduce

1. Load an inline editor in mobile Safari.
2. Add enough text so the editor expands beyond the height of the viewport.
3. Scroll and observe that the inline editor does not stick to the top of the viewport.

Expected result

The inline editor should stick to the top of the viewport like it does on desktop and Android browsers.

Actual result

The inline editor is scrolled off screen.

Other details (browser, OS, CKEditor version, installed plugins)

iOS 9, mobile Safari, any configuration: basic, standard, or full.