V-cycle re-requests a denied rendering mode forever — skip denied / non-requestable modes (runtime #1499 denial)

[dfattal]

What happens

With runtime main ≥ d21eb46e2 (displayxr-runtime#1499 / PR #1519), a PRIMARY_STEREO session — which the Unity provider always is — gets xrRequestDisplayRenderingModeDXR for a rendering mode it cannot fill (sim-display's 4-view Quad, mode index 4) answered with XR_SUCCESS plus XrEventDataDisplayModeRequestDeniedDXR (reason = XR_DISPLAY_MODE_DENIAL_REASON_VIEW_CONFIG_CANNOT_FILL_DXR, 6).

Measured on the win box (urp-singlepass-ui, provider #328 DLL, sim-display, V×20 via scancode injection): 0 mode crossings, 18 DENYING rendering mode 4 lines — the V-cycle (DisplayXRInputController.cs:95) computes "next = current + 1", requests 4, is denied, stays on the current mode, and next V requests 4 again. The cycle is stuck. desktop-avatar toggles 2D↔Anaglyph directly and is unaffected.

Fix

• On XR_TYPE_EVENT_DATA_DISPLAY_MODE_REQUEST_DENIED_DXR, advance past the denied index (or rebuild the cycle list from xrEnumerateDisplayRenderingModesDXR honouring isRequestable).
• Prefer building the cycle list only from modes with viewCount <= 2 (the provider is stereo-fixed — DXR_PROV_MAX_VIEWS 2) so the request is never made; runtime-side complement tracked as displayxr-runtime (see the linked issue) to report such modes isRequestable = XR_FALSE for the session.

Refs

displayxr-runtime#1499, #1519 (denial), displayxr-unity#328 / #329 (where it was measured), displayxr-runtime#1528 (unrelated frame-edge grace measured in the same run).

[dfattal]

Runtime-side complement filed as displayxr-runtime#1532 (report isRequestable = XR_FALSE for modes the session's view configuration cannot fill). The samples fix should not wait on it — skip denied modes on XR_TYPE_EVENT_DATA_DISPLAY_MODE_REQUEST_DENIED_DXR regardless.