Render <turbo-cable-stream-source> as display: none

Since the `<turbo-cable-stream-source>` is a Custom Element, it has a
default `style:` property of `display: inline`. As a visible element, it
participates in the page's layout.

This commit changes the element to always be `display: none` so that it
no longer participates in the layout.

app/assets/javascripts/turbo.js

@@ -4943,7 +4943,18 @@ function walk(obj) {
   }), {});
 }
 
+const template = Object.assign(document.createElement("template"), {
+  innerHTML: "<style>:host { display: none; }</style>"
+});
+
 class TurboCableStreamSourceElement extends HTMLElement {
+  constructor() {
+    super();
+    this.attachShadow({
+      mode: "open"
+    });
+    this.shadowRoot.appendChild(template.content.cloneNode(true));
+  }
   async connectedCallback() {
     connectStreamSource(this);
     this.subscription = await subscribeTo(this.channel, {

app/javascript/turbo/cable_stream_source_element.js

@@ -2,7 +2,18 @@ import { connectStreamSource, disconnectStreamSource } from "@hotwired/turbo"
 import { subscribeTo } from "./cable"
 import snakeize from "./snakeize"
 
+const template = Object.assign(document.createElement("template"), {
+  innerHTML: "<style>:host { display: none; }</style>"
+})
+
 class TurboCableStreamSourceElement extends HTMLElement {
+  constructor() {
+    super()
+
+    this.attachShadow({ mode: "open" })
+    this.shadowRoot.appendChild(template.content.cloneNode(true))
+  }
+
   async connectedCallback() {
     connectStreamSource(this)
     this.subscription = await subscribeTo(this.channel, {

test/system/broadcasts_test.rb

@@ -24,18 +24,18 @@ class BroadcastsTest < ApplicationSystemTestCase
   test "Message broadcasts with renderable: render option" do
     visit messages_path
     wait_for_stream_to_be_connected
-  
+
     assert_broadcasts_text "Test message", to: :messages do |text, target|
       Message.create(content: "Ignored").broadcast_append_to(target, renderable: MessageComponent.new(text))
     end
   end
-  
+
   test "Does not render the layout twice when passed a component" do
     visit messages_path
     wait_for_stream_to_be_connected
-  
+
     Message.create(content: "Ignored").broadcast_append_to(:messages, renderable: MessageComponent.new("test"))
-  
+
     assert_selector("title", count: 1, visible: false, text: "Dummy")
   end