refactor: only assess performance fees on non-delegated gains

contracts/Vault.vy

@@ -1606,7 +1606,20 @@ def report(gain: uint256, loss: uint256, _debtPayment: uint256) -> uint256:
         self._reportLoss(msg.sender, loss)
 
     # Assess both management fee and performance fee, and issue both as shares of the vault
-    self._assessFees(msg.sender, gain)
+    total_strategy_debt: uint256 = self.strategies[msg.sender].totalDebt
+    cached_strategy_delegatedAssets: uint256 = self._strategy_delegatedAssets[msg.sender]
+    if cached_strategy_delegatedAssets == 0:
+        self._assessFees(msg.sender, gain)
+
+    elif total_strategy_debt > cached_strategy_delegatedAssets:
+        # NOTE: Only assess fees on the non-delegated part of the invested assests
+        adjusted_gain: uint256 = (
+            gain * total_strategy_debt / (total_strategy_debt - cached_strategy_delegatedAssets)
+        )
+        self._assessFees(msg.sender, adjusted_gain)
+
+    else:
+        self._assessFees(msg.sender, 0)
 
     # Returns are always "realized gains"
     self.strategies[msg.sender].totalGain += gain
@@ -1647,7 +1660,7 @@ def report(gain: uint256, loss: uint256, _debtPayment: uint256) -> uint256:
 
     # Update cached value of delegated assets
     #   (used to properly account for mgmt fee in `_assessFees`)
-    self.delegatedAssets -= self._strategy_delegatedAssets[msg.sender]
+    self.delegatedAssets -= cached_strategy_delegatedAssets
     delegatedAssets: uint256 = Strategy(msg.sender).delegatedAssets()
     self.delegatedAssets += delegatedAssets
     self._strategy_delegatedAssets[msg.sender] = delegatedAssets