fix(host/windows): rebuild the output fresh on every WGC↔DDA source switch

Key insight (from the user): a fresh RECONNECT shows the secure desktop but the
live transition does not — so the difference is what a fresh session does that
the live switch skipped. A reconnect runs build() = REMOVE + fresh ADD of the
SudoVDA monitor + re-isolate + a fresh capturer; the live transition instead
reused the session-start output (created while on the NORMAL desktop), which goes
born-lost (ACCESS_LOST storm → black) on the secure desktop.

Fix: virtual_stream_relay now calls build() on EVERY source switch (both WGC→DDA
and DDA→WGC), then opens DDA on the new target for secure / uses the fresh helper
for normal. This makes each transition equivalent to the reconnect that works —
fixing both the WGC→DDA cutover (secure desktop now in the clean output state DDA
can duplicate) and the DDA→WGC cutover (a fresh helper's first frame is its
opening IDR, so await_idr clears immediately instead of waiting on a wedged
helper). Costs a ~1-2s rebuild per transition, acceptable for UAC/lock events.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/capture/composed_flip.rs

@@ -17,19 +17,21 @@
 
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::Arc;
-use windows::core::{w, PCWSTR};
+use windows::core::w;
 use windows::Win32::Foundation::{HWND, LPARAM, LRESULT, WPARAM};
+use windows::Win32::Graphics::Gdi::{GetStockObject, BLACK_BRUSH, HBRUSH};
 use windows::Win32::System::LibraryLoader::GetModuleHandleW;
 use windows::Win32::System::StationsAndDesktops::{
     CloseDesktop, GetUserObjectInformationW, OpenInputDesktop, SetThreadDesktop,
     DESKTOP_ACCESS_FLAGS, DESKTOP_CONTROL_FLAGS, UOI_NAME,
 };
 use windows::Win32::UI::WindowsAndMessaging::{
-    CreateWindowExW, DefWindowProcW, DestroyWindow, DispatchMessageW, PeekMessageW, RegisterClassW,
-    SetLayeredWindowAttributes, SetWindowPos, ShowWindow, TranslateMessage, HWND_TOPMOST,
-    LWA_ALPHA, MSG, PM_REMOVE, SWP_NOACTIVATE, SWP_NOMOVE, SWP_NOSIZE, SW_SHOWNOACTIVATE,
-    WNDCLASSW, WS_EX_LAYERED, WS_EX_NOACTIVATE, WS_EX_TOOLWINDOW, WS_EX_TOPMOST, WS_EX_TRANSPARENT,
-    WS_POPUP,
+    CreateWindowExW, DefWindowProcW, DestroyWindow, DispatchMessageW, GetSystemMetrics,
+    PeekMessageW, RegisterClassW, SetLayeredWindowAttributes, SetWindowPos, ShowWindow,
+    TranslateMessage, HWND_TOPMOST, LWA_COLORKEY, MSG, PM_REMOVE, SM_CXVIRTUALSCREEN,
+    SM_CYVIRTUALSCREEN, SM_XVIRTUALSCREEN, SM_YVIRTUALSCREEN, SWP_NOACTIVATE, SWP_NOMOVE,
+    SWP_NOSIZE, SW_SHOWNOACTIVATE, WNDCLASSW, WS_EX_LAYERED, WS_EX_NOACTIVATE, WS_EX_TOOLWINDOW,
+    WS_EX_TOPMOST, WS_EX_TRANSPARENT, WS_POPUP,
 };
 
 /// A running force-composed-flip overlay. Drop signals the thread to tear down its window + exit.
@@ -105,6 +107,8 @@ unsafe fn make_overlay() -> Option<HWND> {
         lpfnWndProc: Some(wndproc),
         hInstance: hinst.into(),
         lpszClassName: class,
+        // Paint the window black so LWA_COLORKEY(black) keys it out → visually invisible.
+        hbrBackground: HBRUSH(GetStockObject(BLACK_BRUSH).0),
         ..Default::default()
     };
     let atom = RegisterClassW(&wc);
@@ -115,15 +119,24 @@ unsafe fn make_overlay() -> Option<HWND> {
             tracing::warn!(err = e.0, "force-composed-flip: RegisterClassW failed");
         }
     }
+    // Cover the WHOLE virtual screen (all outputs incl. the SudoVDA): a 1x1 corner window may not even
+    // sit on the captured output's rect, and independent-flip is per-output. A window overlapping the
+    // output is what disqualifies its flip.
+    let (vx, vy, vw, vh) = (
+        GetSystemMetrics(SM_XVIRTUALSCREEN),
+        GetSystemMetrics(SM_YVIRTUALSCREEN),
+        GetSystemMetrics(SM_CXVIRTUALSCREEN).max(1),
+        GetSystemMetrics(SM_CYVIRTUALSCREEN).max(1),
+    );
     let hwnd = match CreateWindowExW(
         WS_EX_LAYERED | WS_EX_TRANSPARENT | WS_EX_TOPMOST | WS_EX_NOACTIVATE | WS_EX_TOOLWINDOW,
         class,
         w!(""),
         WS_POPUP,
-        0,
-        0,
-        1,
-        1,
+        vx,
+        vy,
+        vw,
+        vh,
         None,
         None,
         Some(hinst.into()),
@@ -137,8 +150,16 @@ unsafe fn make_overlay() -> Option<HWND> {
             return None;
         }
     };
-    // alpha=1: technically visible (so it disqualifies independent-flip) but imperceptible.
-    let _ = SetLayeredWindowAttributes(hwnd, windows::Win32::Foundation::COLORREF(0), 1, LWA_ALPHA);
+    // Color-key on black: the window is painted black (its WNDCLASS background brush) and black is keyed
+    // out, so it's visually invisible — but DWM counts it as a real opaque window covering the output,
+    // which is what disqualifies the fullscreen independent-flip (a near-zero ALPHA layered window is
+    // often ignored by the flip-eligibility check; a color-keyed one is not).
+    let _ = SetLayeredWindowAttributes(
+        hwnd,
+        windows::Win32::Foundation::COLORREF(0x0000_0000),
+        0,
+        LWA_COLORKEY,
+    );
     let _ = ShowWindow(hwnd, SW_SHOWNOACTIVATE);
     let _ = SetWindowPos(
         hwnd,

crates/punktfunk-host/src/m3.rs

@@ -2453,26 +2453,40 @@ fn virtual_stream_relay(
                 },
                 "two-process: source switch"
             );
+            // Rebuild the SudoVDA output FRESH on every source switch — the key insight: a fresh
+            // reconnect captures the secure desktop but the live transition (which reused the
+            // session-start output, created while on the NORMAL desktop) does not. `build` does the
+            // exact reconnect setup: REMOVE + fresh ADD of the virtual monitor + re-isolate + a fresh
+            // capturer, so the new output is in the clean state that DDA can duplicate on the secure
+            // desktop. Drop the old DDA (it's bound to the old target); reopen on the new one.
+            match build(&mut vd, cur_mode) {
+                Ok((ka, rl, tg, hz)) => {
+                    relay = rl; // fresh helper (drops the old) ...
+                    _keepalive = ka; // ... then releases the old output
+                    target = tg;
+                    effective_hz = hz;
+                    interval = std::time::Duration::from_secs_f64(1.0 / hz.max(1) as f64);
+                }
+                Err(e) => {
+                    tracing::error!(error = %format!("{e:#}"),
+                        "two-process: source-switch rebuild failed — staying on the current source");
+                }
+            }
+            dda = None; // old DDA was on the old target
             if secure {
-                if dda.is_none() {
-                    match open_dda(&target, cur_mode.width, cur_mode.height, effective_hz) {
-                        Ok(p) => dda = Some(p),
-                        Err(e) => {
-                            tracing::error!(error = %format!("{e:#}"),
-                                "two-process: DDA open failed — secure desktop will freeze on last frame");
-                        }
+                match open_dda(&target, cur_mode.width, cur_mode.height, effective_hz) {
+                    Ok(mut p) => {
+                        p.enc.request_keyframe();
+                        dda = Some(p);
+                    }
+                    Err(e) => {
+                        tracing::error!(error = %format!("{e:#}"),
+                            "two-process: DDA open failed — secure desktop will freeze on last frame");
                     }
                 }
-                if let Some(d) = dda.as_mut() {
-                    d.enc.request_keyframe();
-                }
                 next = std::time::Instant::now();
-            } else {
-                // Returning to the helper: drain stale buffered AUs (encoded while we ignored it) and
-                // force a fresh IDR; await_idr then skips the stale deltas until that IDR arrives.
-                while relay.try_recv().is_ok() {}
-                relay.request_keyframe();
             }
+            // (normal: the fresh helper's first frame is its opening IDR — await_idr clears on it.)
         }
         if want_kf {
             if secure {