From cacc757cef60eaaca6d1cb7eacd1d1ecc45a8874 Mon Sep 17 00:00:00 2001
From: Quentin Gliech <quenting@element.io>
Date: Fri, 7 Aug 2026 12:07:30 +0200
Subject: [PATCH] Emit a `modulepreload` for dynamically imported chunks in
 `include_asset`

Now that the translations are lazily imported JS chunks rather than plain JSON
assets, `include_asset('locales/<lang>.json')` resolves to a manifest entry
flagged `isDynamicEntry`. Those must not be evaluated eagerly with a `<script>`
tag, so emit the same `modulepreload` hint we already use for imported chunks,
integrity included, and drop the now-unreachable top-level JSON preload.
---
 crates/spa/src/vite.rs            | 11 ++++++++++-
 crates/templates/src/functions.rs | 28 ++++++++++++++++------------
 2 files changed, 26 insertions(+), 13 deletions(-)

diff --git a/crates/spa/src/vite.rs b/crates/spa/src/vite.rs
index b2d7c0c2a..8a56b0097 100644
--- a/crates/spa/src/vite.rs
+++ b/crates/spa/src/vite.rs
@@ -1,3 +1,4 @@
+// Copyright 2025, 2026 Element Creations Ltd.
 // Copyright 2024, 2025 New Vector Ltd.
 // Copyright 2023, 2024 The Matrix.org Foundation C.I.C.
 //
@@ -30,7 +31,6 @@ pub struct ManifestEntry {
     #[expect(dead_code)]
     is_entry: Option<bool>,
 
-    #[expect(dead_code)]
     is_dynamic_entry: Option<bool>,
 
     imports: Option<Vec<Utf8PathBuf>>,
@@ -90,6 +90,7 @@ pub struct Asset<'a> {
     file_type: FileType,
     name: &'a Utf8Path,
     integrity: Option<&'a str>,
+    is_dynamic_entry: bool,
 }
 
 impl<'a> Asset<'a> {
@@ -101,6 +102,7 @@ impl<'a> Asset<'a> {
             file_type,
             name,
             integrity,
+            is_dynamic_entry: entry.is_dynamic_entry.unwrap_or(false),
         })
     }
 
@@ -116,6 +118,13 @@ impl<'a> Asset<'a> {
         self.file_type
     }
 
+    /// Whether this asset is a chunk which is only reachable through a dynamic
+    /// `import()`, and therefore must not be evaluated eagerly
+    #[must_use]
+    pub fn is_dynamic_entry(&self) -> bool {
+        self.is_dynamic_entry
+    }
+
     /// Get the integrity HTML tag attribute, with a leading space, if any
     #[must_use]
     pub fn integrity_attr(&self) -> String {
diff --git a/crates/templates/src/functions.rs b/crates/templates/src/functions.rs
index ea47be14d..16515572e 100644
--- a/crates/templates/src/functions.rs
+++ b/crates/templates/src/functions.rs
@@ -485,6 +485,20 @@ impl Object for IncludeAsset {
         // We'll accumulate the output in this string
         let mut output = String::new();
         match main.file_type() {
+            mas_spa::FileType::Script if main.is_dynamic_entry() => {
+                // Chunks which are only reachable through a dynamic `import()`
+                // (like the translation files) must not be evaluated eagerly,
+                // we only hint the browser to fetch them
+                let integrity = main.integrity_attr();
+                let src = main.src(assets_base);
+                if tracker.mark_preloaded(&src) {
+                    writeln!(
+                        output,
+                        r#"<link rel="modulepreload" href="{src}" crossorigin="anonymous"{integrity} />"#
+                    )
+                    .unwrap();
+                }
+            }
             mas_spa::FileType::Script => {
                 let integrity = main.integrity_attr();
                 let src = main.src(assets_base);
@@ -508,14 +522,6 @@ impl Object for IncludeAsset {
                 }
             }
 
-            mas_spa::FileType::Json => {
-                // When a JSON is included at the top level (a translation), we preload it
-                let src = main.src(assets_base);
-                if tracker.mark_preloaded(&src) {
-                    writeln!(output, r#"<link rel="preload" href="{src}" as="fetch" />"#).unwrap();
-                }
-            }
-
             file_type => {
                 return Err(Error::new(
                     ErrorKind::InvalidOperation,
@@ -560,10 +566,8 @@ impl Object for IncludeAsset {
                     }
                 }
                 mas_spa::FileType::Woff | mas_spa::FileType::Woff2 | mas_spa::FileType::Json => {
-                    // Skip pre-loading fonts and JSON (translations) as it will
-                    // lead to many wasted preloads. For translations, we only
-                    // include them as preload if they are included on the
-                    // top-level
+                    // Skip pre-loading fonts and raw JSON assets, as it will
+                    // lead to many wasted preloads
                 }
             }
         }