Fix for Chieftain resist nodes applying effects twice due to Foulborn Choir of the Storm

spec/System/TestDefence_spec.lua

@@ -358,6 +358,26 @@ describe("TestDefence", function()
 		build.skillsTab.socketGroupList = {}
 	end)
 
+	it("foulborn resistance conversion remains stable across recalculation", function()
+		build.configTab.input.enemyIsBoss = "None"
+		build.configTab.input.customMods = "\z
+		+300 to fire resistance\n\z
+		modifiers to fire resistance also apply to cold and lightning resistances at 50% of their value\n\z
+		mana is increased by 100% of overcapped lightning resistance\n\z
+		"
+		build.configTab:BuildModList()
+		runCallback("OnFrame")
+
+		assert.are.equals(90, build.calcsTab.calcsOutput.LightningResistTotal)
+		assert.are.equals(15, build.calcsTab.calcsOutput.LightningResistOverCap)
+
+		build.configTab:BuildModList()
+		runCallback("OnFrame")
+
+		assert.are.equals(90, build.calcsTab.calcsOutput.LightningResistTotal)
+		assert.are.equals(15, build.calcsTab.calcsOutput.LightningResistOverCap)
+	end)
+
 	-- fun part
 	it("armoured max hits", function()
 		build.configTab.input.enemyIsBoss = "None"
@@ -1177,4 +1197,4 @@ describe("TestDefence", function()
 		assert.are.equals(0, floor(poolsRemaining.Life))
 		assert.are.equals(0, floor(poolsRemaining.OverkillDamage))
 	end)
-end)
+end)

src/Modules/CalcDefence.lua

@@ -532,7 +532,7 @@ function calcs.resistances(actor)
 			end
 		end
 	end
-	
+
 	for _, resFrom in ipairs(resistTypeList) do
 		local res
 		for _, resTo in ipairs(resistTypeList) do

src/Modules/CalcPerform.lua

@@ -3189,8 +3189,16 @@ function calcs.perform(env, skipEHP)
 
 	-- Foulborn Choir of the Storm, needs to be after main auras (incase purity of lightning/elements auras) but before extra auras (Radiant Faith)
 	if modDB:Flag(nil, "ManaIncreasedByOvercappedLightningRes") then
-		-- Calclate resistances for ManaIncreasedByOvercappedLightningRes
-		calcs.resistances(env.player)
+		-- Calculate resistances for ManaIncreasedByOvercappedLightningRes without mutating conversion mods on the player ModDB.
+		local tempResistActor = {
+			modDB = new("ModDB", modDB),
+			output = output,
+			activeSkillList = env.player.activeSkillList,
+			enemy = env.player.enemy,
+		}
+		tempResistActor.player = tempResistActor
+		tempResistActor.modDB.actor = tempResistActor
+		calcs.resistances(tempResistActor)
 		-- Set the life/mana reservations again as we now have increased mana from overcapped lightning resistance
 		doActorLifeMana(env.player)
 		doActorLifeManaReservation(env.player, true)