Fix change output detection using IVK instead of static address list

- Use try_sapling_note_decryption with IVKs to detect wallet-owned outputs
- Correctly identifies change sent to diversified addresses
- Prevents change outputs from appearing as external recipients in tx history

lib/src/lightwallet.rs

@@ -1452,11 +1452,12 @@ pub fn scan_full_tx(&self, tx: &Transaction, height: i32, datetime: u64) {
         // Also scan the output to see if it can be decoded with our OutgoingViewKey
         // If it can, then we sent this transaction, so we should be able to get
         // the memo and value for our records
-        // First, collect all our z addresses, to check for change
-        // Collect z addresses
-        let z_addresses = self.zkeys.read().unwrap().iter().map( |zk| {
-            encode_payment_address(self.config.hrp_sapling_address(), &zk.zaddress)
-        }).collect::<HashSet<String>>();
+
+        // Collect IVKs to detect change outputs to diversified addresses
+        let ivks_for_change: Vec<_> = self.zkeys.read().unwrap().iter()
+            .map(|zk| zk.extfvk.fvk.vk.ivk())
+            .collect();
+
         // Search all ovks that we have
         let ovks: Vec<_> = self.zkeys.read().unwrap().iter()
         .map(|zk| zk.extfvk.fvk.ovk.clone())
@@ -1472,13 +1473,18 @@ pub fn scan_full_tx(&self, tx: &Transaction, height: i32, datetime: u64) {
                     Some((note, payment_address, memo)) => {
                         let address = encode_payment_address(self.config.hrp_sapling_address(), 
                                         &payment_address);
-                         // Check if this is change, and if it also doesn't have a memo, don't add 
-                        // to the outgoing metadata. 
-                        // If this is change (i.e., funds sent to ourself) AND has a memo, then
-                        // presumably the users is writing a memo to themself, so we will add it to 
-                        // the outgoing metadata, even though it might be confusing in the UI, but hopefully
-                        // the user can make sense of it. 
-                        if z_addresses.contains(&address) && memo.to_utf8().is_none() {
+
+                        // Check if this output belongs to our wallet using IVK decryption.
+                        // This correctly detects change sent to diversified addresses,
+                        // not just the wallet's default z-address.
+                        let epk_prime = output.ephemeral_key.as_prime_order(&JUBJUB).unwrap();
+                        let is_to_self = ivks_for_change.iter().any(|ivk| {
+                            try_sapling_note_decryption(ivk, &epk_prime, &output.cmu, &output.enc_ciphertext).is_some()
+                        });
+
+                        // If this is change (funds to ourself) without a memo, skip it.
+                        // If the user sent a memo to themselves, keep it in outgoing metadata.
+                        if is_to_self && memo.to_utf8().is_none() {
                             continue;
                         }
                         // Update the WalletTx