diff --git a/src/xrt/compositor/metal/comp_metal_compositor.m b/src/xrt/compositor/metal/comp_metal_compositor.m
index d261b96ecf..c20da9b87f 100644
--- a/src/xrt/compositor/metal/comp_metal_compositor.m
+++ b/src/xrt/compositor/metal/comp_metal_compositor.m
@@ -4018,6 +4018,21 @@ - (BOOL)wantsUpdateLayer
 				}
 			}
 
+			// #1788: window-space layers BLEND over the projection content,
+			// same rule as D3D11/D3D12, vk_native and GL (#1786/#1787):
+			// SOURCE_ALPHA_BIT clear means premultiplied bytes (colour src
+			// ONE), set means straight (colour src SOURCE_ALPHA); colour dst
+			// and the alpha "over" (ONE / ONE_MINUS_SOURCE_ALPHA) are shared,
+			// so atlas alpha survives for transparent sessions (#225/#1784).
+			// The blended projection pipelines carry exactly those factors.
+			// Deliberately NOT comp_layer_blend_mode() (#1599): see the
+			// comment in comp_d3d11_renderer.cpp's window-space draw. And
+			// never projection_pipeline — its REPLACE is for the base blit.
+			bool ws_premultiplied =
+			    (layer->data.flags & XRT_LAYER_COMPOSITION_BLEND_TEXTURE_SOURCE_ALPHA_BIT) == 0;
+			id<MTLRenderPipelineState> ws_pipeline =
+			    ws_premultiplied ? c->proj_premult_pipeline : c->proj_straight_pipeline;
+
 			// Source UV sub-rect (default to full texture if not specified)
 			struct xrt_normalized_rect nr = ws->sub.norm_rect;
 			if (nr.w <= 0.0f || nr.h <= 0.0f) {
@@ -4060,7 +4075,7 @@ - (BOOL)wantsUpdateLayer
 				}
 				[encoder setViewport:vp];
 
-				[encoder setRenderPipelineState:c->projection_pipeline];
+				[encoder setRenderPipelineState:ws_pipeline];
 				[encoder setDepthStencilState:c->depth_stencil_state];
 				[encoder setFragmentTexture:src_tex atIndex:0];
 				[encoder setFragmentSamplerState:c->sampler_linear atIndex:0];
diff --git a/tests/tests_comp_layer_view_camera.cpp b/tests/tests_comp_layer_view_camera.cpp
index 88c4f004f3..1b1c4332ff 100644
--- a/tests/tests_comp_layer_view_camera.cpp
+++ b/tests/tests_comp_layer_view_camera.cpp
@@ -1138,7 +1138,9 @@ TEST_CASE("comp_layer_blend_mode: no backend reimplements the blend rule (#1621)
 	    // channel D3D11's is (its own premul/straight decision, deliberately
 	    // not comp_layer_blend_mode() -- see comp_d3d11_renderer.cpp).
 	    {"gl/comp_gl_compositor.cpp", 1},
-	    {"metal/comp_metal_compositor.m", 0},
+	    // #1788: Metal's one allowance is the same Local2D / window-space
+	    // channel as D3D11/D3D12/GL (#1581, #1786).
+	    {"metal/comp_metal_compositor.m", 1},
 	    // The Local2D / window-space channel, above.
 	    {"d3d11/comp_d3d11_renderer.cpp", 1},
 	    /*