Replace WebpackRTLPlugin with a more-compatible bespoke solution.

config/RtlCssPlugin.ts

@@ -0,0 +1,50 @@
+import path from 'path'
+import rtlcss from 'rtlcss'
+import type { Compiler } from 'webpack'
+import type { ConfigureOptions } from 'rtlcss'
+
+export interface WebpackRTLPluginOptions {
+	entries: Set<string>
+	rtlcssConfig: ConfigureOptions
+	transformFilename: (sourceFilename: string) => string
+}
+
+export class RtlCssPlugin {
+	private readonly options: WebpackRTLPluginOptions
+
+	private static readonly defaultOptions: WebpackRTLPluginOptions = {
+		entries: new Set(),
+		rtlcssConfig: {},
+		transformFilename: sourceFilename =>
+			`${path.parse(sourceFilename).name}-rtl.css`
+	}
+
+	constructor(options: Partial<WebpackRTLPluginOptions>) {
+		this.options = { ...RtlCssPlugin.defaultOptions, ...options }
+	}
+
+	apply(compiler: Compiler) {
+		const { Compilation, sources: { ConcatSource } } = compiler.webpack
+		const rtlcssProcessor = rtlcss.configure(this.options.rtlcssConfig)
+
+		compiler.hooks.compilation.tap(RtlCssPlugin.name, compilation => {
+			compilation.hooks.processAssets.tap({
+				name: RtlCssPlugin.name,
+				stage: Compilation.PROCESS_ASSETS_STAGE_ADDITIONAL
+			}, assets => {
+				for (const chunk of compilation.chunks) {
+					if (chunk.name && !this.options.entries.has(chunk.name)) {
+						for (const sourceFilename of chunk.files) {
+							if ('.css' === path.extname(sourceFilename)) {
+								const rtlFilename = this.options.transformFilename(sourceFilename)
+								const ltrCss = assets[sourceFilename].source()
+								const rtlCss = rtlcssProcessor.process(ltrCss).css
+								compilation.emitAsset(rtlFilename, new ConcatSource(rtlCss), { sourceFilename })
+							}
+						}
+					}
+				}
+			})
+		})
+	}
+}

config/webpack-css.ts

@@ -5,10 +5,10 @@ import autoprefixer from 'autoprefixer'
 import hexrgba from 'postcss-hexrgba'
 import MiniCssExtractPlugin from 'mini-css-extract-plugin'
 import RemoveEmptyScriptsPlugin from 'webpack-remove-empty-scripts'
-import WebpackRTLPlugin from 'webpack-rtl-plugin'
 import { glob } from 'glob'
-import type { Config as PostCssConfig } from 'postcss-load-config'
+import { RtlCssPlugin } from './RtlCssPlugin'
 import type { Configuration, EntryObject } from 'webpack'
+import type { Config as PostCssConfig } from 'postcss-load-config'
 
 const postcssOptions: PostCssConfig = {
 	plugins: [
@@ -99,9 +99,8 @@ export const cssWebpackConfig: Configuration = {
 						.replace(/-css\.css$/, '.css') :
 					'[name].css'
 		}),
-		new WebpackRTLPlugin({
-			test: /^(?<filename>edit|manage)\.css$/,
-			filename: [/(?<ext>\.css)/i, '-rtl$1']
+		new RtlCssPlugin({
+			entries: new Set(['manage-css', 'edit-css'])
 		})
 	]
 }