URenderPipeline: Fix blend state with default/mc shaders on 1.17-1.20.6, third try

src/main/kotlin/gg/essential/universal/render/URenderPipeline.kt

@@ -51,6 +51,10 @@ import net.minecraft.client.renderer.WorldVertexBufferUploader
 //$$ import net.minecraft.client.render.Shader
 //$$ import java.util.function.Supplier
 //#endif
+//#if MC<12100
+//$$ import net.minecraft.client.gl.GlBlendState
+//$$ import java.lang.invoke.MethodHandles
+//#endif
 //#else
 import gg.essential.universal.shader.GlShader
 import net.minecraft.client.renderer.vertex.VertexFormatElement
@@ -129,9 +133,23 @@ class URenderPipeline private constructor(
         // (On 1.21+, MC's `GlBlendState` is still broken, but `ShaderProgram` no longer applies it)
         //#if MC>=11700 && MC<12100
         //$$ RenderSystem.getShader()?.let { shaderProgram ->
+        //$$     // Capture actual desired blend state to restore afterwards
         //$$     val prevBlendState = BlendState.active()
-        //$$     shaderProgram.bind()
-        //$$     shaderProgram.unbind()
+        //$$     // This seemingly pointless change of active blend state here is necessary because of another bug in
+        //$$     // GlBlendState: It'll only update its global `activeBlendState` when the blend state is enabled or disabled
+        //$$     // but not when merely the blend function or one of its parameters is changed. So we need to make sure
+        //$$     // we enable a blend state which is the opposite of the blend state of the shader before we enable that one.
+        //$$     // We need to do two calls because we don't know the state of the current `activeBlendState`.
+        //$$     if (shaderProgram.glBlendState.isBlendDisabled) {
+        //$$         BlendState.DISABLED.mc.enable()
+        //$$         BlendState.ALPHA.mc.enable()
+        //$$     } else {
+        //$$         BlendState.ALPHA.mc.enable()
+        //$$         BlendState.DISABLED.mc.enable()
+        //$$     }
+        //$$     // Enable shader's GlBlendState so it becomes the `GlBlendState.activeBlendState`
+        //$$     shaderProgram.glBlendState.enable()
+        //$$     // Restore desired blend state
         //$$     prevBlendState.activateWithoutChangingMcBlendState()
         //$$ }
         //#endif
@@ -522,6 +540,22 @@ class URenderPipeline private constructor(
         val isRequired = false
         //#endif
 
+        //#if MC>=11700 && MC<12100
+        //$$ private val Shader_glBlendState: (Shader) -> GlBlendState by lazy {
+        //$$     try {
+        //$$         val field = Shader::class.java.declaredFields
+        //$$             .first { it.type == GlBlendState::class.java }
+        //$$         field.isAccessible = true
+        //$$         val handle = MethodHandles.lookup().unreflectGetter(field)
+        //$$         return@lazy { shader: Shader -> handle.invoke(shader) as GlBlendState }
+        //$$     } catch (e: Exception) {
+        //$$         e.printStackTrace()
+        //$$         return@lazy { GlBlendState() }
+        //$$     }
+        //$$ }
+        //$$ private val Shader.glBlendState: GlBlendState get() = Shader_glBlendState(this)
+        //#endif
+
         //#if !STANDALONE
         //#if MC>=12105
         //$$ @JvmStatic