On mount ConnectedRoot immediately reported the iframe height to the
parent page while the app was still showing the global preloader, so the
parent shrank the iframe from its initial size to ~63px and then grew it
back step by step as content rendered. On pages with many comments this
reads as the widget blinking several times before loading (reported for
radio-t.com). The June frontend dependency refresh (#2091) shifted
render/effect timing enough to make the premature measurement happen on
every load rather than only on slow connections.

Move the height reporting into Root and start it in the setState callback
that replaces the preloader with real content: the first height message
now always describes rendered content, the iframe never shrinks below it,
and subsequent ResizeObserver updates only grow the frame as comments
arrive. Also adds the previously missing observer disconnect on unmount.

Verified by instrumenting the embed with a height-message listener:
master sent 63px then 316px on an empty test page (v1.16.1 sent a single
316px); with this fix the first message is 316px again.