WebGLRenderer: Honor alpha definition in custom context.

[Mugen87]

Fixed #23495.

Description

Ensures the _alpha is properly set if a custom rendering context is passed to WebGLRenderer.

[mrdoob]

What do you think about doing this instead?

diff --git a/src/renderers/WebGLRenderer.js b/src/renderers/WebGLRenderer.js
index e7d52b3292..4a51ea07a1 100644
--- a/src/renderers/WebGLRenderer.js
+++ b/src/renderers/WebGLRenderer.js
@@ -59,7 +59,6 @@ function WebGLRenderer( parameters = {} ) {
 	const _canvas = parameters.canvas !== undefined ? parameters.canvas : createCanvasElement(),
 		_context = parameters.context !== undefined ? parameters.context : null,
 
-		_alpha = parameters.alpha !== undefined ? parameters.alpha : false,
 		_depth = parameters.depth !== undefined ? parameters.depth : true,
 		_stencil = parameters.stencil !== undefined ? parameters.stencil : true,
 		_antialias = parameters.antialias !== undefined ? parameters.antialias : false,
@@ -300,7 +299,7 @@ function WebGLRenderer( parameters = {} ) {
 		materials = new WebGLMaterials( properties );
 		renderLists = new WebGLRenderLists();
 		renderStates = new WebGLRenderStates( extensions, capabilities );
-		background = new WebGLBackground( _this, cubemaps, state, objects, _alpha, _premultipliedAlpha );
+		background = new WebGLBackground( _this, cubemaps, state, objects, _premultipliedAlpha );
 		shadowMap = new WebGLShadowMap( _this, objects, capabilities );
 
 		bufferRenderer = new WebGLBufferRenderer( _gl, extensions, info, capabilities );
diff --git a/src/renderers/webgl/WebGLBackground.js b/src/renderers/webgl/WebGLBackground.js
index c50575b740..2f8d03d74e 100644
--- a/src/renderers/webgl/WebGLBackground.js
+++ b/src/renderers/webgl/WebGLBackground.js
@@ -7,10 +7,10 @@ import { Mesh } from '../../objects/Mesh.js';
 import { ShaderLib } from '../shaders/ShaderLib.js';
 import { cloneUniforms } from '../shaders/UniformsUtils.js';
 
-function WebGLBackground( renderer, cubemaps, state, objects, alpha, premultipliedAlpha ) {
+function WebGLBackground( renderer, cubemaps, state, objects, premultipliedAlpha ) {
 
 	const clearColor = new Color( 0x000000 );
-	let clearAlpha = alpha === true ? 0 : 1;
+	let clearAlpha = renderer.getContextAttributes().alpha === true ? 0 : 1;
 
 	let planeMesh;
 	let boxMesh;

[Mugen87]

It will work for a custom context with alpha: true. However it breaks the current usage of alpha: true when passed as a ctor parameter to WebGLRenderer.

three.js always creates a rendering context with alpha: true since last release. However, the alpha parameter is still used for picking a different clear alpha value in WebGLBackground. With the above code, this does not happen anymore.

[mrdoob]

Ah... That's true 👍

[mrdoob]

Thanks!