release: v0.24.0 — ScaleChangedEvent (ADR-059)

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.24.0] - 2026-08-01
+
+### Added
+
+- **`ScaleChangedEvent`** (ADR-059, gogpu#409) — runtime DPI/scale factor change event. Emitted when window moves between monitors with different DPI or OS DPI settings change. Carries `ScaleFactor`, `Width`, `Height`. Event ordering: `ScaleChangedEvent` before `ResizeEvent` (cause before effect, winit pattern). Enterprise research: winit, Qt6, SDL3, Flutter — all emit separate DPI event.
+
 ## [0.23.0] - 2026-07-30
 
 ### Added

input_event.go

@@ -86,3 +86,23 @@ type ResizeEvent struct {
 }
 
 func (ResizeEvent) inputEventTag() {}
+
+// ScaleChangedEvent represents a DPI scale factor change.
+//
+// Emitted when a window moves between monitors with different DPI (e.g.,
+// Retina 2x → FullHD 1x), or when the OS DPI setting changes at runtime.
+//
+// ScaleFactor is the new display scale (1.0 = standard, 2.0 = Retina/HiDPI).
+// Width and Height are the new logical size in DIP — may change during DPI
+// transition because the OS adjusts the window to preserve visual size.
+//
+// Event ordering: ScaleChangedEvent is emitted BEFORE ResizeEvent (cause
+// before effect, winit pattern). Consumers should update their rendering
+// scale before processing the new dimensions.
+type ScaleChangedEvent struct {
+	ScaleFactor float64
+	Width       int
+	Height      int
+}
+
+func (ScaleChangedEvent) inputEventTag() {}

input_event_test.go

@@ -13,13 +13,14 @@ func TestInputEventInterface(t *testing.T) {
 		FocusEvent{Focused: true},
 		FocusEvent{Focused: false},
 		ResizeEvent{Width: 800, Height: 600},
+		ScaleChangedEvent{ScaleFactor: 2.0, Width: 800, Height: 600},
 		PointerEvent{Type: PointerDown, X: 100, Y: 200},
 		PointerEvent{Type: PointerMove, X: 150, Y: 250},
 		ScrollEvent{DeltaY: -3.0, DeltaMode: ScrollDeltaLine},
 	}
 
-	if len(events) != 9 {
-		t.Fatalf("expected 9 events, got %d", len(events))
+	if len(events) != 10 {
+		t.Fatalf("expected 10 events, got %d", len(events))
 	}
 
 	for i, ev := range events {
@@ -37,6 +38,7 @@ func TestInputEventTypeSwitch(t *testing.T) {
 		CharEvent{Char: 'x'},
 		FocusEvent{Focused: true},
 		ResizeEvent{Width: 1024, Height: 768},
+		ScaleChangedEvent{ScaleFactor: 2.0, Width: 800, Height: 600},
 	}
 
 	counts := map[string]int{}
@@ -54,12 +56,14 @@ func TestInputEventTypeSwitch(t *testing.T) {
 			counts["focus"]++
 		case ResizeEvent:
 			counts["resize"]++
+		case ScaleChangedEvent:
+			counts["scale"]++
 		default:
 			t.Errorf("unexpected event type: %T", ev)
 		}
 	}
 
-	for _, name := range []string{"key", "pointer", "scroll", "char", "focus", "resize"} {
+	for _, name := range []string{"key", "pointer", "scroll", "char", "focus", "resize", "scale"} {
 		if counts[name] != 1 {
 			t.Errorf("expected 1 %s event, got %d", name, counts[name])
 		}
@@ -107,6 +111,27 @@ func TestResizeEventFields(t *testing.T) {
 	}
 }
 
+func TestScaleChangedEventFields(t *testing.T) {
+	ev := ScaleChangedEvent{ScaleFactor: 2.0, Width: 800, Height: 600}
+	if ev.ScaleFactor != 2.0 {
+		t.Errorf("ScaleFactor = %v, want 2.0", ev.ScaleFactor)
+	}
+	if ev.Width != 800 || ev.Height != 600 {
+		t.Errorf("got %dx%d, want 800x600", ev.Width, ev.Height)
+	}
+}
+
+func TestScaleChangedEventImplementsInputEvent(t *testing.T) {
+	var ev InputEvent = ScaleChangedEvent{ScaleFactor: 1.5, Width: 1024, Height: 768}
+	se, ok := ev.(ScaleChangedEvent)
+	if !ok {
+		t.Fatal("ScaleChangedEvent does not implement InputEvent")
+	}
+	if se.ScaleFactor != 1.5 {
+		t.Errorf("ScaleFactor = %v, want 1.5", se.ScaleFactor)
+	}
+}
+
 func TestPointerEventImplementsInputEvent(t *testing.T) {
 	var ev InputEvent = PointerEvent{Type: PointerDown, X: 50, Y: 100}
 	pe, ok := ev.(PointerEvent)