FEATURE: maintain preview theme, while previewing

This means you can browse around in preview mode without losing the theme.
At any point you can refresh page and maintain the preview theme.

app/assets/javascripts/discourse/initializers/live-development.js.es6

@@ -7,6 +7,28 @@ export default {
   initialize(container) {
     const messageBus = container.lookup('message-bus:main');
 
+    if (window.history && window.location.search.indexOf("?preview_theme_key=") === 0) {
+      // force preview theme key to always be carried along
+      const themeKey = window.location.search.slice(19).split('&')[0];
+      if (themeKey.match(/^[a-z0-9-]+$/i)) {
+        const patchState = function(f) {
+          const patched = window.history[f];
+
+          window.history[f] = function(stateObj, name, url) {
+            if (url.indexOf("preview_theme_key=") === -1) {
+              const joiner = url.indexOf("?") === -1 ? "?" : "&";
+              url = `${url}${joiner}preview_theme_key=${themeKey}`;
+            }
+
+            return patched.call(window.history, stateObj, name, url);
+          };
+        };
+        patchState("replaceState");
+        patchState("pushState");
+      }
+
+    }
+
     // Custom header changes
     $('header.custom').each(function() {
       const header = $(this);

app/controllers/admin/themes_controller.rb

@@ -7,8 +7,7 @@ class Admin::ThemesController < Admin::AdminController
 
   def preview
     @theme = Theme.find(params[:id])
-
-    redirect_to path("/"), flash: { preview_theme_key: @theme.key }
+    redirect_to path("/?preview_theme_key=#{@theme.key}")
   end
 
   def upload_asset

app/controllers/application_controller.rb

@@ -315,7 +315,7 @@ class ApplicationController < ActionController::Base
     resolve_safe_mode
     return if request.env[NO_CUSTOM]
 
-    theme_key = flash[:preview_theme_key]
+    theme_key = request[:preview_theme_key]
 
     user_option = current_user&.user_option