From a8930e40830f72526bb0191b308a850b379d277d Mon Sep 17 00:00:00 2001
From: Joe Haddad <joe.haddad@zeit.co>
Date: Thu, 10 Sep 2020 11:12:57 -0400
Subject: [PATCH] Allow components to import Global CSS from npm

---
 .../build/webpack/config/blocks/css/index.ts  | 77 +++++++++++++------
 1 file changed, 54 insertions(+), 23 deletions(-)

diff --git a/packages/next/build/webpack/config/blocks/css/index.ts b/packages/next/build/webpack/config/blocks/css/index.ts
index 5843509d6c431..ff32c0a553a93 100644
--- a/packages/next/build/webpack/config/blocks/css/index.ts
+++ b/packages/next/build/webpack/config/blocks/css/index.ts
@@ -154,7 +154,7 @@ export const css = curry(async function css(
     loader({
       oneOf: [
         {
-          test: [regexCssModules, regexSassModules].filter(Boolean),
+          test: [regexCssModules, regexSassModules],
           use: {
             loader: 'error-loader',
             options: {
@@ -171,13 +171,13 @@ export const css = curry(async function css(
       loader({
         oneOf: [
           {
-            test: [regexCssGlobal, regexSassGlobal].filter(Boolean),
+            test: [regexCssGlobal, regexSassGlobal],
             use: require.resolve('next/dist/compiled/ignore-loader'),
           },
         ],
       })
     )
-  } else if (ctx.customAppFile) {
+  } else {
     fns.push(
       loader({
         oneOf: [
@@ -188,28 +188,59 @@ export const css = curry(async function css(
             // See https://github.com/webpack/webpack/issues/6571
             sideEffects: true,
             test: regexCssGlobal,
-            issuer: { and: [ctx.customAppFile] },
+            // We only allow Global CSS to be imported anywhere in the
+            // application if it comes from node_modules. This is a best-effort
+            // heuristic that makes a safety trade-off for better
+            // interoperability with npm packages that require CSS. Without
+            // this ability, the component's CSS would have to be included for
+            // the entire app instead of specific page where it's required.
+            include: { and: [/node_modules/] },
+            // Global CSS is only supported in the user's application, not in
+            // node_modules.
+            issuer: {
+              and: [ctx.rootDirectory],
+              not: [/node_modules/],
+            },
             use: getGlobalCssLoader(ctx, postCssPlugins),
           },
         ],
       })
     )
-    fns.push(
-      loader({
-        oneOf: [
-          {
-            // A global Sass import always has side effects. Webpack will tree
-            // shake the Sass without this option if the issuer claims to have
-            // no side-effects.
-            // See https://github.com/webpack/webpack/issues/6571
-            sideEffects: true,
-            test: regexSassGlobal,
-            issuer: { and: [ctx.customAppFile] },
-            use: getGlobalCssLoader(ctx, postCssPlugins, sassPreprocessors),
-          },
-        ],
-      })
-    )
+
+    if (ctx.customAppFile) {
+      fns.push(
+        loader({
+          oneOf: [
+            {
+              // A global CSS import always has side effects. Webpack will tree
+              // shake the CSS without this option if the issuer claims to have
+              // no side-effects.
+              // See https://github.com/webpack/webpack/issues/6571
+              sideEffects: true,
+              test: regexCssGlobal,
+              issuer: { and: [ctx.customAppFile] },
+              use: getGlobalCssLoader(ctx, postCssPlugins),
+            },
+          ],
+        })
+      )
+      fns.push(
+        loader({
+          oneOf: [
+            {
+              // A global Sass import always has side effects. Webpack will tree
+              // shake the Sass without this option if the issuer claims to have
+              // no side-effects.
+              // See https://github.com/webpack/webpack/issues/6571
+              sideEffects: true,
+              test: regexSassGlobal,
+              issuer: { and: [ctx.customAppFile] },
+              use: getGlobalCssLoader(ctx, postCssPlugins, sassPreprocessors),
+            },
+          ],
+        })
+      )
+    }
   }
 
   // Throw an error for Global CSS used inside of `node_modules`
@@ -217,7 +248,7 @@ export const css = curry(async function css(
     loader({
       oneOf: [
         {
-          test: [regexCssGlobal, regexSassGlobal].filter(Boolean),
+          test: [regexCssGlobal, regexSassGlobal],
           issuer: { and: [/node_modules/] },
           use: {
             loader: 'error-loader',
@@ -235,7 +266,7 @@ export const css = curry(async function css(
     loader({
       oneOf: [
         {
-          test: [regexCssGlobal, regexSassGlobal].filter(Boolean),
+          test: [regexCssGlobal, regexSassGlobal],
           use: {
             loader: 'error-loader',
             options: {
@@ -294,7 +325,7 @@ export const css = curry(async function css(
           // selector), this assumption is required to code-split CSS.
           //
           // If this warning were to trigger, it'd be unactionable by the user,
-          // but also not valid -- so we disable it.
+          // but likely not valid -- so we disable it.
           ignoreOrder: true,
         })
       )