add plugin to avoid too many css requests (#62530)

### What?

Reduces the number of css chunks per entrypoint by merging chunks.

### Why?

In larger application the total CSS deduplication approach falls short
in a sense of it's causing way too many separate chunks, which results
in way to many requests. That affects page load performance.


Closes PACK-2598

packages/next/src/build/webpack-config.ts

@@ -83,6 +83,7 @@ import {
   createAppRouterApiAliases,
 } from './create-compiler-aliases'
 import { hasCustomExportOutput } from '../export/utils'
+import { MergeCssChunksPlugin } from './webpack/plugins/merge-css-chunks-plugin'
 
 type ExcludesFalse = <T>(x: T | false) => x is T
 type ClientEntries = {
@@ -1877,6 +1878,7 @@ export default async function getBaseWebpackConfig(
         new NextFontManifestPlugin({
           appDir,
         }),
+      !dev && isClient && new MergeCssChunksPlugin(),
       !dev &&
         isClient &&
         new (require('./webpack/plugins/telemetry-plugin').TelemetryPlugin)(

packages/next/src/build/webpack/plugins/merge-css-chunks-plugin.ts

@@ -0,0 +1,80 @@
+import type { Chunk, ChunkGraph, Compiler } from 'webpack'
+
+const PLUGIN_NAME = 'MergeCssChunksPlugin'
+
+/**
+ * Css chunks smaller than this size will be merged with other css chunks.
+ */
+const MIN_CSS_CHUNK_SIZE = 30 * 1024
+/**
+ * When merging css chunks it will select an number N where the total size is just bigger than this size.
+ * Exception: N must be at least 2 even if the size is already bigger than this size.
+ */
+const TARGET_CSS_CHUNK_SIZE = 60 * 1024
+/**
+ * When an entrypoint has more css chunks than this number it merge the smallest ones to try to stay below that number.
+ * Exception: When there are more than twice as much css chunks that larger than MAX_CSS_CHUNK_SIZE it will only half the number of css chunks.
+ */
+const MAX_CSS_CHUNKS = 15
+
+function isCssChunk(chunkGraph: ChunkGraph, chunk: Chunk): boolean {
+  for (const mod of chunkGraph.getChunkModulesIterable(chunk)) {
+    return mod.type.startsWith('css')
+  }
+  return false
+}
+
+export class MergeCssChunksPlugin {
+  public apply(compiler: Compiler) {
+    compiler.hooks.thisCompilation.tap(PLUGIN_NAME, (compilation) => {
+      let once = false
+      compilation.hooks.optimizeChunks.tap(
+        {
+          name: PLUGIN_NAME,
+          stage: 20,
+        },
+        () => {
+          if (once) {
+            return
+          }
+          once = true
+          const chunkGraph = compilation.chunkGraph
+          let changed = false
+          for (const [, entrypoint] of compilation.entrypoints) {
+            const cssChunks = entrypoint.chunks
+              .filter((chunk) => isCssChunk(chunkGraph, chunk))
+              .map((chunk) => [chunk, chunkGraph.getChunkSize(chunk)] as const)
+            cssChunks.sort((a, b) => b[1] - a[1])
+
+            // We want have at most MAX_CSS_CHUNKS chunks.
+            // When we start merging at startMergingIndex this would half the number of chunks after that index.
+            const startMergingIndex = 2 * MAX_CSS_CHUNKS - cssChunks.length
+
+            // We select small chunks and chunks after the index
+            const selectedCssChunks = cssChunks.filter(
+              ([, size], i) =>
+                size < MIN_CSS_CHUNK_SIZE || i >= startMergingIndex
+            )
+            while (selectedCssChunks.length >= 2) {
+              const [biggest, bigSize] = selectedCssChunks.shift()!
+              let mergedSize = bigSize
+              do {
+                const [smallest, size] = selectedCssChunks.pop()!
+                if (chunkGraph.canChunksBeIntegrated(biggest, smallest)) {
+                  chunkGraph.integrateChunks(biggest, smallest)
+                  compilation.chunks.delete(smallest)
+                  mergedSize += size
+                  changed = true
+                }
+              } while (
+                selectedCssChunks.length > 0 &&
+                mergedSize < TARGET_CSS_CHUNK_SIZE
+              )
+            }
+          }
+          return changed
+        }
+      )
+    })
+  }
+}