OpenClaw Fixes Chat Transcript Resize Jumps

This is a small but meaningful UX repair: preserving scroll position is essential for any agent interface built around long, dynamic transcripts.

• –Retains stale row measurements instead of collapsing offscreen messages to default estimates
• –Keeps mid-transcript readers on the same message during pane resizing
• –Preserves end-pinned sessions through the existing scroll-to-end behavior
• –Adds an end-to-end test covering 120 mixed-width messages across multiple viewport changes
• –The PR also includes an unrelated port-test retry that reviewers flagged as weakening the original assertion