Add default properties caching and update matching logic

- Implemented a caching mechanism for default properties per class in `rbx_dom_lua/init.lua`, allowing for efficient retrieval of encoded default values, including inherited defaults.
- Updated the matching logic in `Reconciler/matching.lua` to account for properties present only on the Studio side, ensuring accurate comparisons against default values during reconciliation.
- Added new test cases to validate the handling of default-omitted properties, enhancing the robustness of the reconciliation process.

These changes improve the efficiency and accuracy of property matching, addressing issues related to default values in the reconciliation workflow.

plugin/rbx_dom_lua/init.lua

@@ -6,6 +6,43 @@ local PropertyDescriptor = require(script.PropertyDescriptor)
 -- Returns nil if the class doesn't exist in the database
 -- The properties table maps property names to metadata (scriptability, serialization status)
 -- To get a full PropertyDescriptor for encoding, use findCanonicalPropertyDescriptor
+-- Cache for default properties per class (includes inherited defaults).
+local defaultPropertiesCache: { [string]: { [string]: any } } = {}
+
+-- Returns a table mapping property names to their encoded default values
+-- for a given class, including inherited defaults from parent classes.
+-- Subclass overrides take precedence. Results are cached per class.
+local function findDefaultProperties(className: string): { [string]: any }
+	local cached = defaultPropertiesCache[className]
+	if cached then
+		return cached
+	end
+
+	local defaults: { [string]: any } = {}
+	local currentClassName: string? = className
+
+	repeat
+		local currentClass = database.Classes[currentClassName]
+		if currentClass == nil then
+			break
+		end
+
+		local classDefaults = currentClass.DefaultProperties
+		if classDefaults then
+			for propName, encodedValue in classDefaults do
+				if defaults[propName] == nil then
+					defaults[propName] = encodedValue
+				end
+			end
+		end
+
+		currentClassName = currentClass.Superclass
+	until currentClassName == nil
+
+	defaultPropertiesCache[className] = defaults
+	return defaults
+end
+
 local function findClassDescriptor(className)
 	local classData = database.Classes[className]
 	if classData == nil then
@@ -111,6 +148,7 @@ return {
 	writeProperty = writeProperty,
 	findCanonicalPropertyDescriptor = findCanonicalPropertyDescriptor,
 	findClassDescriptor = findClassDescriptor,
+	findDefaultProperties = findDefaultProperties,
 	Error = Error,
 	EncodedValue = require(script.EncodedValue),
 }

plugin/src/Reconciler/matching.lua

@@ -330,6 +330,48 @@ function Matching._countOwnDiffs(vInst: any, studioInstance: Instance): number
 		end
 	end
 
+	-- Properties present only on the Studio side (not in virtual).
+	-- Syncback strips default-valued properties from model files, so virtual
+	-- instances may omit properties that Studio has at non-default values.
+	-- Without this check, a virtual instance missing e.g. Face scores equally
+	-- against all Studio instances in the group, causing wrong pairings.
+	local defaults = RbxDom.findDefaultProperties(vInst.ClassName)
+	if defaults then
+		for propName, encodedDefault in defaults do
+			if propName == "Tags" or propName == "Attributes" then
+				continue
+			end
+
+			-- Skip properties already compared from the virtual side
+			if vProps and vProps[propName] ~= nil then
+				continue
+			end
+
+			-- Skip Ref/UniqueId types (not comparable during hydration)
+			local ty = next(encodedDefault)
+			if ty == "Ref" or ty == "UniqueId" then
+				continue
+			end
+
+			-- pcall guards against unsupported types (e.g. SharedString)
+			local pcallOk, decodeOk, defaultValue = pcall(RbxDom.EncodedValue.decode, encodedDefault)
+			if not pcallOk or not decodeOk then
+				continue
+			end
+
+			local readOk, studioValue = pcall(function()
+				return (studioInstance :: any)[propName]
+			end)
+			if not readOk then
+				continue
+			end
+
+			if not trueEquals(defaultValue, studioValue) then
+				cost += 1
+			end
+		end
+	end
+
 	-- Tags: count individual adds/removes
 	local vTags = vInst.Properties and vInst.Properties.Tags
 	if vTags then