Add support for message history pagination, preserve scroll in rendering, and improve draft/message workflows.

public/js/api.js

@@ -82,18 +82,19 @@ function normalizeError(error, scope, response) {
     });
 }
 
-export async function request(url, data, method = "POST", options = {}) {
+async function bufferedRequest(url, {method, body, headers}, options = {}, asBlob = false) {
     const scope = requestScope(options);
     let response;
     try {
         response = await abortable(() => fetch(url, {
             method,
             credentials: "same-origin",
-            headers: {"Accept": "application/json", "Content-Type": "application/json"},
-            body: data == null ? undefined : JSON.stringify(data),
+            headers,
+            body,
             signal: scope.signal
         }), scope.signal);
-        return await readJson(response, scope.signal);
+        if (!asBlob || !response.ok) return await readJson(response, scope.signal);
+        return {blob: await abortable(() => response.blob(), scope.signal)};
     } catch (error) {
         return normalizeError(error, scope, response).toJSON();
     } finally {
@@ -101,6 +102,17 @@ export async function request(url, data, method = "POST", options = {}) {
     }
 }
 
+export function request(url, data, method = "POST", options = {}) {
+    return bufferedRequest(url, {method, headers: {"Accept": "application/json", "Content-Type": "application/json"},
+        body: data == null ? undefined : JSON.stringify(data)}, options);
+}
+
+// Let fetch set the multipart boundary; share deadlines, cancellation and errors.
+export const multipart = (url, form, options = {}) => bufferedRequest(url,
+    {method: "POST", headers: {"Accept": "application/json"}, body: form}, {timeoutMs: 120000, ...options});
+export const download = (url, options = {}) => bufferedRequest(url,
+    {method: "GET", headers: {"Accept": "*/*"}}, {timeoutMs: 120000, ...options}, true);
+
 export const get = (url, options) => request(url, null, "GET", options);
 export const post = (url, data, options) => request(url, data, "POST", options);
 export const put = (url, data, options) => request(url, data, "PUT", options);