Rewind + "Summarize from here" drops pre-rewind messages instead of preserving them

Summary

Using Rewind → "Summarize from here" with auto-compact disabled caused all pre-rewind messages to be dropped entirely from the conversation, while only the post-rewind messages were summarized. The dialog text states "Messages after this point will be summarized", implying pre-rewind content should be preserved verbatim. That is not what happened.

Additionally, after the summarization completed, the rewind history was cleared. I could not rewind again to recover the lost content.

Expected behavior

Per the dialog text "Messages after this point will be summarized":

• Pre-rewind messages: preserved verbatim in context
• Post-rewind messages: replaced by a summary

Actual behavior

• Pre-rewind messages (~20% of 1M context, an entire workstream): dropped, not preserved, not summarized
• Post-rewind messages (~20% of context): summarized correctly
• Rewind history cleared. "Restore conversation" on the same rewind point was no longer available; session appeared to start fresh

Environment

• Auto-compact: disabled (confirmed via /config, see screenshot)
• Context window: 1M (Opus 4.6)
• Platform: macOS (darwin 25.3.0)
• Session ID: 267c208d-a402-4442-b57c-d55e50f07acf

Impact

Lost ~20% of 1M context of substantive work (an onboarding workstream for a new contractor, including email drafts, document writes, Q&A). Recovered manually by parsing the session JSONL on disk, which still contained the full history. So the underlying log is intact; the bug is in how "Summarize from here" mutated the conversation state.

Reproduction (best guess)

1. Have a long session (~40%+ of context used).
2. Rewind to a point roughly halfway through.
3. Choose "Summarize from here" instead of "Restore conversation".
4. Observe that pre-rewind content is gone rather than preserved.

Unclear whether total session length, rewind depth, or something else is the trigger. Only observed once so far.

Notes

Prior uses of "Summarize from here" (per user recollection) worked as the dialog describes: pre-rewind preserved, post-rewind summarized. This session was the first time the behavior inverted.