* fix(app): fill short collapsed chat histories
Collapsed whole turns can leave the initial viewport short even when older history exists. Continue paging through measured pages before revealing a fresh chat, while preserving visible history and the reading anchor.
* fix(app): settle padded and stalled chat history
Measure the native history edge using the same content area as viewport filling, and bound child history requests so retry can start after a stalled fetch.
* fix(app): adapt viewport filling to the current timeline
* fix(app): resume history filling after switching tabs