diff --git a/src/components/include/request.ts b/src/components/include/request.ts
index efcb818c..8985b6a9 100644
--- a/src/components/include/request.ts
+++ b/src/components/include/request.ts
@@ -4,20 +4,26 @@ interface IncludeFile {
   html: string;
 }
 
-const includeFiles = new Map<string, Promise<IncludeFile>>();
+const includeFiles = new Map<string, IncludeFile | Promise<IncludeFile>>();
 
 /** Fetches an include file from a remote source. Caching is enabled so the origin is only pinged once. */
 export function requestInclude(src: string, mode: 'cors' | 'no-cors' | 'same-origin' = 'cors'): Promise<IncludeFile> {
-  if (includeFiles.has(src)) {
-    return includeFiles.get(src)!;
+  const prev = includeFiles.get(src);
+  if (prev !== undefined) {
+    // Promise.resolve() transparently unboxes prev if it was a promise.
+    return Promise.resolve(prev);
   }
   const fileDataPromise = fetch(src, { mode: mode }).then(async response => {
-    return {
+    const res = {
       ok: response.ok,
       status: response.status,
       html: await response.text()
     };
+    // Replace the cached promise with its result to avoid having buggy browser Promises retain memory as mentioned in #1284 and #1249
+    includeFiles.set(src, res);
+    return res;
   });
+  // Cache the promise to only fetch() once per src
   includeFiles.set(src, fileDataPromise);
   return fileDataPromise;
 }