Switching from Local node back to Remote causes a wallet re-scan

[shanimal08]

Describe the bug

if you're on Local node and it's still syncing/your wallet hasn't scanned through yet, switching to Remote will result in needing to re-scan your wallet from 0

To Reproduce
Steps to reproduce the behavior:

1. be on/switch to Local node
2. switch to Remote node
3. see wallet still in loading :(
4. restart TU, or refresh wallet history
5. see wallet scanning start from 0 again

Expected behavior
your wallet scanning progress should revert back to what it was before you switched to Local node, or - if it was already scanned - not need to scan, certainly not all the way from 0

---

• OS: macOS
• TU v1.6.9

[brianp]

Dug into this and it's an upstream wallet issue in core.

The wallet's UTXO scanner (utxo_scanner_task.rs) keeps a sliding cache of only the last 720 scanned block headers for reorg detection. When it starts a scan round it checks these cached headers against the connected base node to make sure it's still on the same chain.

When switching from Remote (tip ~499k) to a Local node that's still syncing (tip ~349k), all 720 cached blocks are at heights ~498k-499k. They're all above the local node's tip. The scanner sees every cached block as "above current tip" and marks them all as reorged out. With zero valid cache entries left it falls back to scanning from the wallet birthday height.

Switching back to Remote doesn't help because the scan progress was already wiped.

The scanner logic treats "all cached blocks are above the node's tip" the same as "actual chain reorg". These are very different situations — one means the node just hasn't synced that far yet, the other means the chain actually forked. The fix needs to happen in get_last_scanned_block() to skip blocks above the current tip instead of treating them as reorg evidence.

Log evidence from reproduction:

12:50:59 Found 721 cached previously scanned blocks
12:50:59 Scanning complete to current tip (height: 499653)
  -- switched to local node (tip 349396) --
12:51:47 Found 721 cached previously scanned blocks
12:51:47 WARN Reorg detected on base node. Removing scanned blocks from height 498933
12:51:47 WARN No previously scanned block headers found, resuming scan from wallet birthday
12:51:47 Fresh wallet recovery/scanning: Wallet birthday '1265', scanning from block height '38799'

[brianp]

I'm going to leave this open for now, but it's essentially wont-fix. The wallet may or may not make this change, and we're swapping the wallet out anyway. The new wallet operates differently and may not experience this issue. But we should check once it's finished.