Present the workspace on the display the idle Miku occupies

Sources/MikuCodeApp/Companion/AppPresentation.swift

@@ -235,6 +235,27 @@ enum DisplayFrameSelector {
     }
 }
 
+/// Decides where the reused workspace window must be placed before each
+/// presentation. The window is created once, but the idle Miku panel can move
+/// to another display between opens (screen-parameter changes rebind it to the
+/// display under the pointer) — without this, the workspace reopens on
+/// whichever display it was first created on.
+enum WorkspaceDisplayPlacement {
+    /// Returns the frame the window should adopt, or nil to keep its current
+    /// frame (same display — preserves the user's manual size/position — or
+    /// mid-full-screen, where reframing fights the system transition).
+    static func frameForPresentation(
+        windowScreenFrame: CGRect?,
+        targetScreenFrame: CGRect,
+        targetVisibleFrame: CGRect,
+        isFullScreen: Bool
+    ) -> CGRect? {
+        guard !isFullScreen else { return nil }
+        guard windowScreenFrame != targetScreenFrame else { return nil }
+        return targetVisibleFrame
+    }
+}
+
 enum PresentationLayout {
     static func metrics(in size: CGSize) -> PresentationLayoutMetrics {
         let width = max(1, size.width)

Sources/MikuCodeApp/Companion/MikuPanelCoordinator.swift

@@ -51,13 +51,28 @@ final class MikuPanelCoordinator: NSObject, NSWindowDelegate {
         guard presentation.requestOpen() else { return }
         refreshScreenFrameFromPanel()
         let window = ensureWorkspaceWindow()
+        moveWorkspaceWindowToPanelDisplay(window)
         workspacePresentedAt = ProcessInfo.processInfo.systemUptime
         panel.orderOut(nil)
         guard ordersFront else { return }
         NSApplication.shared.activate(ignoringOtherApps: true)
         window.makeKeyAndOrderFront(nil)
     }
 
+    /// The workspace window is created once and reused across opens; present it
+    /// on the display the idle Miku currently occupies, not wherever it was
+    /// created.
+    private func moveWorkspaceWindowToPanelDisplay(_ window: WorkspaceWindow) {
+        guard let targetScreen = panel.screen ?? NSScreen.main else { return }
+        guard let frame = WorkspaceDisplayPlacement.frameForPresentation(
+            windowScreenFrame: window.screen?.frame,
+            targetScreenFrame: targetScreen.frame,
+            targetVisibleFrame: targetScreen.visibleFrame,
+            isFullScreen: window.styleMask.contains(.fullScreen)
+        ) else { return }
+        window.setFrame(frame, display: true)
+    }
+
     /// Close requested by clicking the workspace Miku. The click that opened the
     /// workspace lands on the same screen region the Miku close target appears
     /// in, and stale/replayed events from that press can fire the close button

Tests/MikuCodeAppTests/MikuCodeAppTests.swift

@@ -133,6 +133,55 @@ final class PresentationLayoutTests: XCTestCase {
         )
     }
 
+    func testWorkspacePresentsOnTheDisplayTheIdleMikuOccupies() {
+        let builtIn = CGRect(x: 0, y: 0, width: 1_728, height: 1_117)
+        let external = CGRect(x: 1_728, y: -152, width: 1_920, height: 1_080)
+        let externalVisible = external.insetBy(dx: 0, dy: 12)
+
+        // Window created on the built-in display, Miku now on the external one:
+        // the presentation must move it to the external display's visible frame.
+        XCTAssertEqual(
+            WorkspaceDisplayPlacement.frameForPresentation(
+                windowScreenFrame: builtIn,
+                targetScreenFrame: external,
+                targetVisibleFrame: externalVisible,
+                isFullScreen: false
+            ),
+            externalVisible
+        )
+
+        // A window that has never been on screen also moves to Miku's display.
+        XCTAssertEqual(
+            WorkspaceDisplayPlacement.frameForPresentation(
+                windowScreenFrame: nil,
+                targetScreenFrame: external,
+                targetVisibleFrame: externalVisible,
+                isFullScreen: false
+            ),
+            externalVisible
+        )
+
+        // Same display: keep the user's manually adjusted frame.
+        XCTAssertNil(
+            WorkspaceDisplayPlacement.frameForPresentation(
+                windowScreenFrame: external,
+                targetScreenFrame: external,
+                targetVisibleFrame: externalVisible,
+                isFullScreen: false
+            )
+        )
+
+        // Never fight an in-flight full-screen transition.
+        XCTAssertNil(
+            WorkspaceDisplayPlacement.frameForPresentation(
+                windowScreenFrame: builtIn,
+                targetScreenFrame: external,
+                targetVisibleFrame: externalVisible,
+                isFullScreen: true
+            )
+        )
+    }
+
     func testAdaptiveMetricsKeepTerminalLargeAndMikuFarRight() {
         let displaySizes = [
             CGSize(width: 1_024, height: 768),