Fix early-return conflation in Prop.set (oldValue + propchange skip)

src/plugins/props/util/Prop.js

@@ -215,19 +215,24 @@ let Self = class Prop {
 			parsedValue = this.spec.convert.call(element, parsedValue);
 		}
 
-		if (this.equals(parsedValue, oldValue)) {
-			return;
+		// Resolve the default lazily so equality and propchange.oldValue match the getter.
+		if (
+			oldValue === undefined &&
+			!this.spec.get &&
+			this.default !== undefined &&
+			typeof this.default !== "function"
+		) {
+			oldValue = this.get(element);
 		}
 
-		element.props[this.name] = parsedValue;
-
 		let change = {
 			element,
 			source,
 			value: parsedValue,
 			oldValue,
 		};
 
+		// Reflect before the equality check: an explicit equal-value write still updates the attribute.
 		if (source === "property") {
 			// Reflect to attribute?
 			if (this.toAttribute) {
@@ -254,6 +259,12 @@ let Self = class Prop {
 			});
 		}
 
+		if (this.equals(parsedValue, oldValue)) {
+			return;
+		}
+
+		element.props[this.name] = parsedValue;
+
 		this.changed(element, change);
 	}
 

test/plugins/props/propchange.js

@@ -132,11 +132,25 @@ export default {
 		{
 			name: "Assigning current default-resolved value is a no-op",
 			run () {
-				this.data.element.v = 42;
-				return this.data.events;
+				let { element, events } = this.data;
+				let before = events.length;
+				element.v = 42;
+				return events.length - before;
 			},
 			arg: { props: { v: { type: Number, default: 42 } } },
-			expect: [["v", 42]],
+			expect: 0,
+		},
+		{
+			name: "First write reports the resolved default as oldValue",
+			run () {
+				let { element } = this.data;
+				let oldValue;
+				element.addEventListener("propchange", e => (oldValue = e.detail.oldValue));
+				element.prop = "new";
+				return oldValue;
+			},
+			arg: { props: { prop: { default: "initial" } } },
+			expect: "initial",
 		},
 		{
 			name: "Three writes fire three propchange events in order, synchronously",