initial transparency

src/types/nativeWindowChrome.ts

@@ -4,4 +4,8 @@ export interface NativeWindowChromeAPI {
   // was invalid or the subclass could not be installed. Windows-only; a no-op
   // elsewhere.
   applyFlatFrame: (nativeWindowHandle: Buffer) => boolean
+  // Accent-policy acrylic blur (SetWindowCompositionAttribute). Unlike the DWM
+  // system backdrop, the blur persists while the window is unfocused and works
+  // on borderless transparent windows. Windows-only; a no-op elsewhere.
+  setAcrylicBlurBehind: (nativeWindowHandle: Buffer, enable: boolean) => boolean
 }

src/types/windowCapabilities.ts

@@ -5,4 +5,5 @@ export interface WindowCapabilities {
   useNativeDragRegions: boolean
   supportsProgrammaticReposition: boolean
   supportsGeometryPersistence: boolean
+  supportsBlurredBackground: boolean
 }

src/types/windowState.ts

@@ -4,10 +4,26 @@ import type { WindowBounds } from './popout'
 export const WINDOW_LOCAL_STATE_FORMAT = 'prism-window-local'
 export const WINDOW_LOCAL_STATE_VERSION = 1
 
+export type WindowBackgroundMode = 'solid' | 'blurred' | 'clear'
+
+export interface WindowBackgroundState {
+  mode: WindowBackgroundMode
+  /** How much of the desktop shows through the app background, 0-100. */
+  transparency: number
+}
+
+export interface WindowBackgroundSnapshot {
+  /** What the user chose (preserved even when the platform can't honor it). */
+  stored: WindowBackgroundState
+  /** What is actually applied after platform capability downgrades. */
+  effective: WindowBackgroundState
+}
+
 export interface PrismWindowLocalStateV1 {
   format: typeof WINDOW_LOCAL_STATE_FORMAT
   version: typeof WINDOW_LOCAL_STATE_VERSION
   mainAlwaysOnTop: boolean
   popoutAlwaysOnTop: Partial<Record<ScopeKind, boolean>>
   nowPlayingConfigWindowBounds?: WindowBounds
+  windowBackground: WindowBackgroundState
 }