diff --git a/packaging/flatpak/io.unom.Punktfunk.yml b/packaging/flatpak/io.unom.Punktfunk.yml
index 3d5b75ed..4aafa891 100644
--- a/packaging/flatpak/io.unom.Punktfunk.yml
+++ b/packaging/flatpak/io.unom.Punktfunk.yml
@@ -93,20 +93,38 @@ finish-args:
   # /usr/lib/extensions/vulkan/gamescope once installed (a one-time, per-Deck step; keep it in the
   # Decky plugin's setup / docs):
   #     flatpak install --user -y flathub org.freedesktop.Platform.VulkanLayer.gamescope//25.08
-  # THREE things are needed, not two (verified live on a Deck OLED — the env vars alone left
-  # hdr10_format=None). (1) VK_ADD_IMPLICIT_LAYER_PATH puts the layer's implicit-layer JSON on the
-  # Vulkan loader's search path (the runtime point mounts the files but not onto the path). (2)
-  # ENABLE_GAMESCOPE_WSI flips the layer's own `enable_environment` gate. (3) The layer, once
-  # loaded, must open a *Wayland* connection to gamescope's private socket ($GAMESCOPE_WAYLAND_DISPLAY
-  # = gamescope-0) to negotiate the HDR10 colorspace via the gamescope_swapchain protocol — but the
-  # Deck runs games as X11 clients (DISPLAY=:1, no WAYLAND_DISPLAY exported), so --socket=wayland
-  # binds nothing and that socket never enters the sandbox. Without it the layer loads, maps, and
-  # silently can't reach the compositor → no HDR10 offered → PQ tone-mapped to SDR, badge dark.
-  # Binding xdg-run/gamescope-0 is the missing half (chiaki-ng does the same). With all three the
-  # surface offers HDR10 and the presenter's existing HDR10 swapchain path engages — no client code
-  # change. Harmless off-Deck: the layer no-ops when there's no gamescope socket to bind.
+  # FOUR things are needed. An earlier revision of this block claimed three and was WRONG: the
+  # fourth is the gate that makes the other three moot, so the Deck sat at hdr10_format=None with
+  # all of (1)-(3) in place, which is exactly the field report ("HDR->SDR" in the stats overlay).
+  # (1) VK_ADD_IMPLICIT_LAYER_PATH puts the layer's implicit-layer JSON on the Vulkan loader's
+  # search path (the runtime point mounts the files but not onto the path). (2) ENABLE_GAMESCOPE_WSI
+  # flips the layer's own `enable_environment` gate. (3) --filesystem=xdg-run/gamescope-0 binds
+  # gamescope's private Wayland socket: the layer must reach the compositor over it to negotiate
+  # HDR10, and the Deck runs games as X11 clients (DISPLAY=:1, no WAYLAND_DISPLAY exported) so
+  # --socket=wayland binds nothing (chiaki-ng does the same). (4) GAMESCOPE_WAYLAND_DISPLAY must be
+  # set INSIDE the sandbox. The layer's `isRunningUnderGamescope()` reads that env var and nothing
+  # else; flatpak does not forward host env, so it arrives unset and the layer's CreateInstance
+  # early-returns before it ever creates a GamescopeInstance. The layer still LOADS and still logs
+  # its generic bits ("Forcing on VK_EXT_swapchain_maintenance1", swapchain destroys), which is why
+  # this reads as working — but no gamescope surface is made, so no HDR10 format is ever appended
+  # and (1)-(3) buy nothing. Measured on a Deck OLED (Galileo, SteamOS 3.8.16) 2026-08-05, client
+  # `--browse`, reading `pf_presenter::vk::setup` "swapchain config":
+  #     unset                    -> no "[Gamescope WSI] Surface state" block at all, hdr10_format=None
+  #     set, hdr_enabled=0       -> "server hdr output enabled: false", hdr10_format=None
+  #     set, hdr_enabled=1       -> "hdr formats exposed to client: true",
+  #                                 hdr10_format=Some(A2B10G10R10_UNORM_PACK32, HDR10_ST2084_EXT)
+  # DXVK_HDR is NOT the gate for us and was ruled out by measurement: the layer forces it OFF for
+  # clients it has already decided to deny, it does not turn HDR on.
+  # Hardcoding `gamescope-0` matches the socket bound just below, and is safe off-Deck both ways:
+  # on a normal Wayland desktop --socket=wayland sets WAYLAND_DISPLAY=wayland-0 inside the sandbox
+  # and the layer bails on the mismatch; on X11-only there is no gamescope socket to connect to, so
+  # it prints one "Bypass layer will be unavailable" line and passes through.
+  # The REMAINING gate is not ours: gamescope's `hdr_enabled` convar (Steam's HDR display setting)
+  # drives the GAMESCOPE_HDR_OUTPUT_FEEDBACK X property the layer reads, and with it off no app on
+  # the Deck gets HDR. See docs — that one is a user/Decky-side step, not a packaging one.
   - --env=VK_ADD_IMPLICIT_LAYER_PATH=/usr/lib/extensions/vulkan/gamescope/share/vulkan/implicit_layer.d
   - --env=ENABLE_GAMESCOPE_WSI=1
+  - --env=GAMESCOPE_WAYLAND_DISPLAY=gamescope-0   # the layer's ONLY "am I under gamescope?" signal
   - --filesystem=xdg-run/gamescope-0   # gamescope's private Wayland socket (HDR negotiation)
 
 build-options: