diff --git a/native/swift/Sources/Deview/Runtime.swift b/native/swift/Sources/Deview/Runtime.swift
index c84ffbdb..794a16b6 100644
--- a/native/swift/Sources/Deview/Runtime.swift
+++ b/native/swift/Sources/Deview/Runtime.swift
@@ -35,6 +35,9 @@ final class Runtime {
     var input = DeviewInput()
     var initialised = false
 
+    /// Keeps App Nap off for as long as the runtime is open: see `open`.
+    private var activity: NSObjectProtocol?
+
     private init() {
         resetInput()
     }
@@ -49,6 +52,16 @@ final class Runtime {
         self.title = title
         initialised = true
 
+        // Out of App Nap for as long as this runs. A napped process has its timers coalesced and
+        // its priority lowered, and this one is a loop driven from outside, waiting on
+        // nextEvent(until:) a frame at a time, with a socket listener beside it: covered, it could
+        // be slow to answer a failing test's inline send and slower to bring its window forward,
+        // and hidden while it owns the queue it still has to answer every send promptly. Idle
+        // sleep stays allowed. The loop only redraws on a change, so staying awake costs a pump.
+        activity = ProcessInfo.processInfo.beginActivity(
+            options: .userInitiatedAllowingIdleSystemSleep,
+            reason: "Reviewing snapshots and answering the queue")
+
         // A hidden start is capture only, and capture draws into a bitmap of its own making. Not
         // touching AppKit at all in that case is what lets the pixel tests run: NSWindow may only
         // be instantiated on the main thread, and a test host runs them on whatever thread it
@@ -287,6 +300,11 @@ final class Runtime {
     }
 
     func shutdown() {
+        if let activity {
+            ProcessInfo.processInfo.endActivity(activity)
+            self.activity = nil
+        }
+
         window?.delegate = nil
         window?.orderOut(nil)
         window?.close()
diff --git a/src/DiffEngineViewer.Mac/runtimes/osx-arm64/native/libdiffengine_viewer.dylib b/src/DiffEngineViewer.Mac/runtimes/osx-arm64/native/libdiffengine_viewer.dylib
index 7a4b62de..05c76421 100644
Binary files a/src/DiffEngineViewer.Mac/runtimes/osx-arm64/native/libdiffengine_viewer.dylib and b/src/DiffEngineViewer.Mac/runtimes/osx-arm64/native/libdiffengine_viewer.dylib differ
diff --git a/src/DiffEngineViewer.Mac/runtimes/osx-x64/native/libdiffengine_viewer.dylib b/src/DiffEngineViewer.Mac/runtimes/osx-x64/native/libdiffengine_viewer.dylib
index 7a4b62de..05c76421 100644
Binary files a/src/DiffEngineViewer.Mac/runtimes/osx-x64/native/libdiffengine_viewer.dylib and b/src/DiffEngineViewer.Mac/runtimes/osx-x64/native/libdiffengine_viewer.dylib differ
diff --git a/todo.md b/todo.md
index 2b01505c..eca38019 100644
--- a/todo.md
+++ b/todo.md
@@ -16,10 +16,3 @@ Viewer model
 - [ ] **Selection columns count one code point to a cell, which wide CJK and combining marks do not take** (verified)
   - Columns now count code points, which fixed the copy and the highlight for characters outside the basic plane: GDI+ draws those one cell wide and ImGui lays out one glyph per code point (`SelectionText.Cells`). Still off: CJK falls back to a font 1.83 cells wide on Windows, a combining mark takes none, and Core Text substitutes fonts with their own widths.
   - Fix: have each head report string positions from its own layout rather than cells, or put every code point on the grid.
-
-
-Native
-
-- [ ] **macOS App Nap can stall the loop while the window is covered** (cannot verify here)
-  - Nothing opts out (no `beginActivity`, `NSAppSleepDisabled` or power assertion), the only wait is `nextEvent(until: now + 1/60)` (`Runtime.swift:248-253`), and a Focus queued by an arriving patch waits for the next managed frame.
-  - Check on a Mac: cover the viewer for a minute, confirm Activity Monitor shows App Nap, then time how long a failing inline test takes to bring it forward against an uncovered window.