Phone Chrome without EXT_color_buffer_float returned null with one message. Rendering to
RGBA16F only needs EXT_color_buffer_half_float, and every other target is already a half, so
only the prepass start (RGBA32F) needed a fallback. In half mode the prepass writes
t*(1-2^-10) - 2^-15, which stays below t after the half rounds to nearest, so pass 1 still
starts at or before the true distance. W_FAR stays exact. onUnavailable names no-webgl2 vs
neither float extension; Scene.startFormat and options.halfStart (force, for tests and the
harness). The standalone page shows the reason, renderer, vendor, UA and extension list.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
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