Display download progress bar when loading profiles from URLs

When loading profiles via from-url or public data sources, the app
previously showed only a decorative animation with no indication of
download progress. This replaces that with a real progress bar using
the ReadableStream API to track bytes received.

- Stream response body via response.body.getReader() instead of
  response.arrayBuffer() to report progress during download
- Pre-allocate buffer when Content-Length is known to avoid 2x
  memory spike; fall back to chunk accumulation when unknown or wrong
- Throttle progress dispatches to ~10/s to avoid excessive renders
- Show determinate bar with "X MB / Y MB" when Content-Length known,
  indeterminate animation with "X MB downloaded" otherwise
- Aggregate progress across parallel downloads in compare mode
- Add ARIA progressbar role, aria-valuetext, and localized aria-label
- Add Fluent localization strings for all user-visible text
- Respect prefers-reduced-motion: reduce

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kim-em

locales/en-US/app.ftl

@@ -836,6 +836,18 @@ ProfileLoaderAnimation--loading-view-not-found = View not found
 
 ProfileRootMessage--title = { -profiler-brand-name }
 ProfileRootMessage--additional = Back to home
+# This string is used as the accessible label for the download progress bar.
+ProfileRootMessage--download-progress-label =
+    .aria-label = Download progress
+# This string is displayed when the total download size is known.
+# Variables:
+#   $receivedSize (String) - Amount of data received so far, e.g. "3.2 MB"
+#   $totalSize (String) - Total download size, e.g. "14.5 MB"
+ProfileRootMessage--download-progress-known = { $receivedSize } / { $totalSize }
+# This string is displayed when the total download size is unknown.
+# Variables:
+#   $receivedSize (String) - Amount of data received so far, e.g. "3.2 MB"
+ProfileRootMessage--download-progress-unknown = { $receivedSize } downloaded
 
 ## Root
 

src/actions/receive-profile.ts

@@ -6,6 +6,7 @@ import {
   fetchProfile,
   getProfileUrlForHash,
   type ProfileOrZip,
+  type DownloadProgress,
   deduceContentType,
   extractJsonFromArrayBuffer,
 } from 'firefox-profiler/utils/profile-fetch';
@@ -994,6 +995,30 @@ export function retrieveProfileFromStore(
   return retrieveProfileOrZipFromUrl(getProfileUrlForHash(hash), initialLoad);
 }
 
+/**
+ * Create a callback that aggregates download progress across multiple parallel
+ * fetches and dispatches the combined totals. Used for compare-mode downloads.
+ */
+function _makeAggregatedProgressDispatcher(
+  dispatch: (receivedBytes: number, totalBytes: number | null) => void
+): (key: string, progress: DownloadProgress) => void {
+  const progressByKey = new Map<string, DownloadProgress>();
+  return (key: string, progress: DownloadProgress) => {
+    progressByKey.set(key, progress);
+    let receivedBytes = 0;
+    let totalBytes: number | null = 0;
+    for (const p of progressByKey.values()) {
+      receivedBytes += p.receivedBytes;
+      if (totalBytes !== null && p.totalBytes !== null) {
+        totalBytes += p.totalBytes;
+      } else {
+        totalBytes = null;
+      }
+    }
+    dispatch(receivedBytes, totalBytes);
+  };
+}
+
 /**
  * Runs a fetch on a URL, and downloads the file. If it's JSON, then it attempts
  * to process the profile. If it's a zip file, it tries to unzip it, and save it
@@ -1012,6 +1037,13 @@ export function retrieveProfileOrZipFromUrl(
         onTemporaryError: (e: TemporaryError) => {
           dispatch(temporaryError(e));
         },
+        onDownloadProgress: ({ receivedBytes, totalBytes }) => {
+          dispatch({
+            type: 'PROFILE_DOWNLOAD_PROGRESS',
+            receivedBytes,
+            totalBytes,
+          });
+        },
       });
 
       switch (response.responseType) {
@@ -1178,6 +1210,16 @@ export function retrieveProfilesToCompare(
   return async (dispatch) => {
     dispatch(waitingForProfileFromUrl());
 
+    const reportAggregatedProgress = _makeAggregatedProgressDispatcher(
+      (receivedBytes, totalBytes) => {
+        dispatch({
+          type: 'PROFILE_DOWNLOAD_PROGRESS',
+          receivedBytes,
+          totalBytes,
+        });
+      }
+    );
+
     try {
       const profilesAndStates = await Promise.all(
         profileViewUrls.map(async (url) => {
@@ -1196,6 +1238,9 @@ export function retrieveProfilesToCompare(
             onTemporaryError: (e: TemporaryError) => {
               dispatch(temporaryError(e));
             },
+            onDownloadProgress: (progress: DownloadProgress) => {
+              reportAggregatedProgress(profileUrl, progress);
+            },
           });
           if (response.responseType !== 'PROFILE') {
             throw new Error('Expected to receive a profile from fetchProfile');

src/components/app/ProfileLoaderAnimation.tsx

@@ -65,12 +65,16 @@ class ProfileLoaderAnimationImpl extends PureComponent<ProfileLoaderAnimationPro
       dataSource === 'from-file'
     );
 
+    const downloadProgress =
+      'downloadProgress' in view ? view.downloadProgress : null;
+
     return (
       <Localized id={message} attrs={{ title: true }} elems={{ a: <span /> }}>
         <ProfileRootMessage
           additionalMessage={this._renderAdditionalMessage()}
           showLoader={showLoader}
           showBackHomeLink={showBackHomeLink}
+          downloadProgress={downloadProgress}
         >{`Untranslated ${message}`}</ProfileRootMessage>
       </Localized>
     );

src/components/app/ProfileRootMessage.css

@@ -43,6 +43,56 @@
   font-size: 12px;
 }
 
+.downloadProgress {
+  margin-top: 16px;
+}
+
+.downloadProgressBarTrack {
+  overflow: hidden;
+  height: 8px;
+  border-radius: 4px;
+  background-color: var(--grey-30);
+}
+
+.downloadProgressBarFill {
+  height: 100%;
+  border-radius: 4px;
+  background-color: var(--blue-60);
+  transition: width 150ms ease-out;
+}
+
+.downloadProgressBarFillIndeterminate {
+  width: 30%;
+  height: 100%;
+  border-radius: 4px;
+  animation: indeterminateProgress 1.5s ease-in-out infinite;
+  background-color: var(--blue-60);
+}
+
+@keyframes indeterminateProgress {
+  0% {
+    transform: translateX(-100%);
+  }
+
+  100% {
+    transform: translateX(433%);
+  }
+}
+
+@media (prefers-reduced-motion: reduce) {
+  .downloadProgressBarFillIndeterminate {
+    width: 100%;
+    animation: none;
+    opacity: 0.5;
+  }
+}
+
+.downloadProgressText {
+  margin-top: 4px;
+  color: var(--grey-50);
+  font-size: 12px;
+}
+
 .loading {
   position: relative;
   height: 40px;

src/components/app/ProfileRootMessage.tsx

@@ -7,18 +7,40 @@ import * as React from 'react';
 
 import './ProfileRootMessage.css';
 
+type DownloadProgressInfo = {
+  readonly receivedBytes: number;
+  readonly totalBytes: number | null;
+};
+
 type Props = {
   readonly title?: string;
   readonly additionalMessage: React.ReactNode;
   readonly showLoader: boolean;
   readonly showBackHomeLink: boolean;
+  readonly downloadProgress?: DownloadProgressInfo | null;
   readonly children: React.ReactNode;
 };
 
+function _formatBytes(bytes: number): string {
+  if (bytes < 1024) {
+    return `${bytes} B`;
+  }
+  if (bytes < 1024 * 1024) {
+    return `${(bytes / 1024).toFixed(1)} KB`;
+  }
+  return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
+}
+
 export class ProfileRootMessage extends React.PureComponent<Props> {
   override render() {
-    const { children, additionalMessage, showLoader, showBackHomeLink, title } =
-      this.props;
+    const {
+      children,
+      additionalMessage,
+      showLoader,
+      showBackHomeLink,
+      downloadProgress,
+      title,
+    } = this.props;
     return (
       <div className="rootMessageContainer">
         <div className="rootMessage">
@@ -29,6 +51,9 @@ export class ProfileRootMessage extends React.PureComponent<Props> {
           <div className="rootMessageText">
             <p>{children}</p>
           </div>
+          {downloadProgress
+            ? this._renderDownloadProgress(downloadProgress)
+            : null}
           {additionalMessage ? (
             <div className="rootMessageAdditional">{additionalMessage}</div>
           ) : null}
@@ -41,7 +66,7 @@ export class ProfileRootMessage extends React.PureComponent<Props> {
               </a>
             </div>
           ) : null}
-          {showLoader ? (
+          {showLoader && !downloadProgress ? (
             <div className="loading">
               <div className="loading-div loading-div-1 loading-row-1" />
               <div className="loading-div loading-div-2 loading-row-2" />
@@ -59,4 +84,63 @@ export class ProfileRootMessage extends React.PureComponent<Props> {
       </div>
     );
   }
+
+  _renderDownloadProgress(
+    downloadProgress: DownloadProgressInfo
+  ): React.ReactNode {
+    const receivedStr = _formatBytes(downloadProgress.receivedBytes);
+    const progressText = downloadProgress.totalBytes
+      ? `${receivedStr} / ${_formatBytes(downloadProgress.totalBytes)}`
+      : `${receivedStr} downloaded`;
+
+    return (
+      <div className="downloadProgress">
+        <Localized
+          id="ProfileRootMessage--download-progress-label"
+          attrs={{ 'aria-label': true }}
+        >
+          <div
+            className="downloadProgressBarTrack"
+            role="progressbar"
+            aria-valuenow={downloadProgress.receivedBytes}
+            aria-valuemin={0}
+            aria-valuemax={downloadProgress.totalBytes ?? undefined}
+            aria-valuetext={progressText}
+            aria-label="Download progress"
+          >
+            {downloadProgress.totalBytes ? (
+              <div
+                className="downloadProgressBarFill"
+                style={{
+                  width: `${Math.min(100, (downloadProgress.receivedBytes / downloadProgress.totalBytes) * 100)}%`,
+                }}
+              />
+            ) : (
+              <div className="downloadProgressBarFillIndeterminate" />
+            )}
+          </div>
+        </Localized>
+        <div className="downloadProgressText">
+          {downloadProgress.totalBytes ? (
+            <Localized
+              id="ProfileRootMessage--download-progress-known"
+              vars={{
+                receivedSize: receivedStr,
+                totalSize: _formatBytes(downloadProgress.totalBytes),
+              }}
+            >
+              {progressText}
+            </Localized>
+          ) : (
+            <Localized
+              id="ProfileRootMessage--download-progress-unknown"
+              vars={{ receivedSize: receivedStr }}
+            >
+              {progressText}
+            </Localized>
+          )}
+        </div>
+      </div>
+    );
+  }
 }

src/reducers/app.ts

@@ -34,6 +34,14 @@ const view: Reducer<AppViewState> = (
       };
     case 'FATAL_ERROR':
       return { phase: 'FATAL_ERROR', error: action.error };
+    case 'PROFILE_DOWNLOAD_PROGRESS':
+      return {
+        phase: 'INITIALIZING',
+        downloadProgress: {
+          receivedBytes: action.receivedBytes,
+          totalBytes: action.totalBytes,
+        },
+      };
     case 'WAITING_FOR_PROFILE_FROM_BROWSER':
     case 'WAITING_FOR_PROFILE_FROM_URL':
     case 'WAITING_FOR_PROFILE_FROM_FILE':

src/test/store/receive-profile.test.ts

@@ -958,6 +958,12 @@ describe('actions/receive-profile', function () {
             message: errorMessage,
           },
         },
+        expect.objectContaining({
+          phase: 'INITIALIZING',
+          downloadProgress: expect.objectContaining({
+            receivedBytes: expect.any(Number),
+          }),
+        }),
         { phase: 'PROFILE_LOADED' },
         { phase: 'DATA_LOADED' },
       ]);
@@ -1082,6 +1088,12 @@ describe('actions/receive-profile', function () {
             message: errorMessage,
           },
         },
+        expect.objectContaining({
+          phase: 'INITIALIZING',
+          downloadProgress: expect.objectContaining({
+            receivedBytes: expect.any(Number),
+          }),
+        }),
         { phase: 'PROFILE_LOADED' },
         { phase: 'DATA_LOADED' },
       ]);

src/types/actions.ts

@@ -436,6 +436,11 @@ type ReceiveProfileAction =
       readonly profileUrl: string | null;
     }
   | { readonly type: 'TRIGGER_LOADING_FROM_URL'; readonly profileUrl: string }
+  | {
+      readonly type: 'PROFILE_DOWNLOAD_PROGRESS';
+      readonly receivedBytes: number;
+      readonly totalBytes: number | null;
+    }
   | {
       readonly type: 'UPDATE_PAGES';
       readonly newPages: PageList;

src/types/state.ts

@@ -125,6 +125,10 @@ export type AppViewState =
         readonly attempt: Attempt | null;
         readonly message: string;
       };
+      readonly downloadProgress?: {
+        readonly receivedBytes: number;
+        readonly totalBytes: number | null;
+      };
     };
 
 export type Phase = AppViewState['phase'];