fix(windows-host): force EXTEND topology so a new IddCx display isn't cloned

A freshly-added IddCx virtual display lands in CLONE/duplicate mode when a
physical display is already active (a laptop panel, an attached monitor): the
cloned output shares that display's source, so the OS never commits a distinct
path for it, never calls ASSIGN_SWAPCHAIN, and capture sees no frames - the
session fails "not an active display path / needs a WDDM GPU to activate" and
tears down with 0 frames (seen live on an Intel-iGPU + NVIDIA-Optimus laptop).

force_extend_topology() applies the EXTEND preset (the programmatic Win+P
"Extend") right after ADD so the IDD comes up as its own active path; the
existing resolve_gdi_name -> set_active_mode -> isolate_displays_ccd bring-up
then proceeds. Idempotent / no-op on a sole-display (headless single-GPU) box,
so it's safe on the path that already worked.

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

crates/punktfunk-host/src/vdisplay/windows/manager.rs

@@ -27,7 +27,8 @@ use windows::Win32::Foundation::{HANDLE, LUID};
 
 use super::{Mode, VirtualOutput};
 use crate::win_display::{
-    isolate_displays_ccd, resolve_gdi_name, restore_displays_ccd, set_active_mode, SavedConfig,
+    force_extend_topology, isolate_displays_ccd, resolve_gdi_name, restore_displays_ccd,
+    set_active_mode, SavedConfig,
 };
 
 /// The per-backend REMOVE key the driver stamps on ADD and consumes on REMOVE. SudoVDA keys monitors by
@@ -326,6 +327,15 @@ impl VirtualDisplayManager {
             }
         });
 
+        // Windows defaults a new IddCx monitor into CLONE mode when a physical display is already
+        // active (a laptop panel, an attached monitor): the cloned IDD shares that display's source, so
+        // the OS never commits a distinct path for it and capture sees no frames. Force EXTEND first so
+        // the IDD comes up as its OWN active path; the resolve loop below then finds it. Idempotent /
+        // no-op on a sole-display box, so it's safe on the headless single-GPU path too.
+        // SAFETY: `force_extend_topology` only calls `SetDisplayConfig` (a CCD topology apply) with no
+        // borrowed caller memory; it runs under the manager `state` lock, the sole topology mutator.
+        unsafe { force_extend_topology() };
+
         // Resolve the capture target. May be None on a GPU-less box (target added but not WDDM-activated);
         // the capture backend re-resolves once a GPU is present.
         let mut gdi_name = None;

crates/punktfunk-host/src/windows/win_display.rs

@@ -21,7 +21,7 @@ use windows::Win32::Devices::Display::{
     DISPLAYCONFIG_GET_ADVANCED_COLOR_INFO, DISPLAYCONFIG_MODE_INFO, DISPLAYCONFIG_PATH_INFO,
     DISPLAYCONFIG_SET_ADVANCED_COLOR_STATE, DISPLAYCONFIG_SOURCE_DEVICE_NAME,
     QDC_ONLY_ACTIVE_PATHS, SDC_ALLOW_CHANGES, SDC_APPLY, SDC_FORCE_MODE_ENUMERATION,
-    SDC_SAVE_TO_DATABASE, SDC_USE_SUPPLIED_DISPLAY_CONFIG,
+    SDC_SAVE_TO_DATABASE, SDC_TOPOLOGY_EXTEND, SDC_USE_SUPPLIED_DISPLAY_CONFIG,
 };
 use windows::Win32::Graphics::Gdi::{
     ChangeDisplaySettingsExW, EnumDisplaySettingsW, CDS_TEST, CDS_UPDATEREGISTRY, DEVMODEW,
@@ -31,6 +31,29 @@ use windows::Win32::Graphics::Gdi::{
 
 use crate::vdisplay::Mode;
 
+/// Force the desktop into EXTEND topology - the programmatic equivalent of the Win+P / DisplaySwitch
+/// "Extend" shortcut. Windows defaults a FRESHLY-ADDED monitor into CLONE/duplicate mode when a
+/// physical display is already active (e.g. a laptop panel): a cloned IddCx output shares the panel's
+/// source, so the OS never commits a distinct path for it, never calls ASSIGN_SWAPCHAIN, and capture
+/// sees no frames (`resolve_gdi_name` stays `None` and the session fails "not an active display path").
+/// Applying the EXTEND preset across the live set of connected displays makes the new IddCx monitor its
+/// OWN active path, so the rest of bring-up (`resolve_gdi_name` -> `set_active_mode` ->
+/// `isolate_displays_ccd`) proceeds. Best-effort + idempotent: a no-op on a single-display (already
+/// sole/extended) box, so it is safe to call unconditionally. `rc == 0` is success.
+pub(crate) unsafe fn force_extend_topology() {
+    // A topology flag with no supplied path/mode arrays tells the OS to recompute + apply that preset
+    // for the currently-connected displays (the same code path DisplaySwitch.exe drives).
+    let rc = SetDisplayConfig(None, None, SDC_APPLY | SDC_TOPOLOGY_EXTEND);
+    if rc == 0 {
+        tracing::info!(
+            "display topology forced to EXTEND (a new IddCx monitor would otherwise be CLONED onto the \
+             existing panel -> no distinct source -> no frames)"
+        );
+    } else {
+        tracing::warn!("display force-EXTEND topology: SetDisplayConfig rc={rc:#x}");
+    }
+}
+
 /// Resolve the `\\.\DisplayN` GDI name for a SudoVDA target id via the CCD API. Returns `None`
 /// until the OS activates the target into the desktop topology (needs a real WDDM GPU; on a
 /// GPU-less box this stays `None` even though ADD succeeded).