ProjectToNdc clipped only the eye half-space (w>MinW, a 2026-06-03 workaround) and left the 4 frustum side planes to the 2D ScreenPolygonClip. When the eye is within a portal's near plane, small-w verts explode under the perspective divide (probe saw NDC (10.2,-67.4)); the 2D clip then collapses to empty -> OutsideView empty -> terrain Skip -> the bluish doorway void. Clip the eye plane + 4 side planes (homogeneous Sutherland-Hodgman) before the divide so NDC is bounded to the screen by construction, matching retail GetClip -> polyClipFinish (clip in clip-space before the divide; pc:432344).
Partial: NOT the full flicker fix. The dominant cause (camera boom drift + viewer-cell flip at boundaries + missing w=0 near-plane clip) is identified and deferred to the next session per the handoff. 2 RED->GREEN tests.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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