From eb667c35bd1c32b3ea2fc5f56326665298131b3a Mon Sep 17 00:00:00 2001
From: Evan Wallace <evan.exe@gmail.com>
Date: Fri, 11 Aug 2023 20:39:34 -0400
Subject: [PATCH] css: try to merge adjacent `@layer` rules together

---
 internal/bundler_tests/bundler_css_test.go    | 34 +++++++++++++++++++
 .../bundler_tests/snapshots/snapshots_css.txt | 33 +++++++++++++++---
 internal/linker/linker.go                     | 21 ++++++++++++
 3 files changed, 84 insertions(+), 4 deletions(-)

diff --git a/internal/bundler_tests/bundler_css_test.go b/internal/bundler_tests/bundler_css_test.go
index 87c26ee4302..dc6d601c26a 100644
--- a/internal/bundler_tests/bundler_css_test.go
+++ b/internal/bundler_tests/bundler_css_test.go
@@ -2537,3 +2537,37 @@ func TestCSSAtLayerBeforeImportBundle(t *testing.T) {
 		},
 	})
 }
+
+func TestCSSAtLayerMergingWithImportConditions(t *testing.T) {
+	css_suite.expectBundled(t, bundled{
+		files: map[string]string{
+			"/entry.css": `
+				@import "a.css" supports(color: first);
+
+				@import "a.css" supports(color: second);
+				@import "b.css" supports(color: second);
+
+				@import "a.css" supports(color: first);
+				@import "b.css" supports(color: first);
+
+				@import "a.css" supports(color: second);
+				@import "b.css" supports(color: second);
+
+				@import "b.css" supports(color: first);
+			`,
+			"/a.css": `
+				@layer a;
+				@import "http://example.com/a.css";
+			`,
+			"/b.css": `
+				@layer b;
+				@import "http://example.com/b.css";
+			`,
+		},
+		entryPaths: []string{"/entry.css"},
+		options: config.Options{
+			Mode:         config.ModeBundle,
+			AbsOutputDir: "/out",
+		},
+	})
+}
diff --git a/internal/bundler_tests/snapshots/snapshots_css.txt b/internal/bundler_tests/snapshots/snapshots_css.txt
index 171ba696589..a7139abdeda 100644
--- a/internal/bundler_tests/snapshots/snapshots_css.txt
+++ b/internal/bundler_tests/snapshots/snapshots_css.txt
@@ -250,8 +250,7 @@ TestCSSAtImportConditionsAtLayerBundleAlternatingLayerInFile
 /* case2.css */
 
 ---------- /out/case3.css ----------
-@layer first;
-@layer last;
+@layer first, last;
 
 /* a.css */
 @layer first {
@@ -289,8 +288,7 @@ TestCSSAtImportConditionsAtLayerBundleAlternatingLayerInFile
 /* case4.css */
 
 ---------- /out/case5.css ----------
-@layer first;
-@layer last;
+@layer first, last;
 
 /* a.css */
 @layer first {
@@ -1614,6 +1612,33 @@ TestCSSAtLayerBeforeImportNoBundle
 @import "b.css";
 @layer layer6.layer7, layer8;
 
+================================================================================
+TestCSSAtLayerMergingWithImportConditions
+---------- /out/entry.css ----------
+@supports (color: first) {
+  @layer a;
+}
+@import "http://example.com/a.css" supports(color: first);
+@import "http://example.com/a.css" supports(color: second);
+@import "http://example.com/b.css" supports(color: second);
+@import "http://example.com/b.css" supports(color: first);
+@supports (color: second) {
+  @layer a, b;
+}
+
+/* a.css */
+@supports (color: first) {
+  @layer b;
+}
+
+/* a.css */
+
+/* b.css */
+
+/* b.css */
+
+/* entry.css */
+
 ================================================================================
 TestCSSEntryPoint
 ---------- /out.css ----------
diff --git a/internal/linker/linker.go b/internal/linker/linker.go
index 5968f9d03e3..2385735e835 100644
--- a/internal/linker/linker.go
+++ b/internal/linker/linker.go
@@ -3703,6 +3703,27 @@ func (c *linkerContext) findImportedFilesInCSSOrder(entryPoints []uint32) (order
 			wipOrder = append(wipOrder, entry)
 		}
 
+		order, wipOrder = wipOrder, order[:0]
+	}
+
+	// Finally, merge adjacent "@layer" rules with identical conditions together.
+	{
+		didClone := -1
+		for _, entry := range order {
+			if entry.kind == cssImportLayers && len(wipOrder) > 0 {
+				prevIndex := len(wipOrder) - 1
+				prev := wipOrder[prevIndex]
+				if prev.kind == cssImportLayers && importConditionsAreEqual(prev.conditions, entry.conditions) {
+					if didClone != prevIndex {
+						didClone = prevIndex
+						prev.layers = append([][]string{}, prev.layers...)
+					}
+					wipOrder[prevIndex].layers = append(prev.layers, entry.layers...)
+					continue
+				}
+			}
+			wipOrder = append(wipOrder, entry)
+		}
 		order = wipOrder
 	}