Indicate who and when the room was set to `world_readable`

server/lib/matrix-utils/fetch-room-data.js

@@ -210,9 +210,15 @@ const fetchRoomData = traceFunction(async function (
   }
 
   let historyVisibility;
+  let historyVisibilityEventMeta;
   if (stateHistoryVisibilityResDataOutcome.reason === undefined) {
     const { data } = stateHistoryVisibilityResDataOutcome.value;
     historyVisibility = data?.content?.history_visibility;
+    historyVisibilityEventMeta = {
+      historyVisibility,
+      sender: data?.sender,
+      originServerTs: data?.origin_server_ts,
+    };
   }
 
   let roomCreationTs;
@@ -240,6 +246,7 @@ const fetchRoomData = traceFunction(async function (
     canonicalAlias,
     avatarUrl,
     historyVisibility,
+    historyVisibilityEventMeta,
     roomCreationTs,
     predecessorRoomId,
     predecessorLastKnownEventId,

server/routes/room-routes.js

@@ -833,7 +833,10 @@ router.get(
     if (!allowedToViewRoom) {
       throw new StatusError(
         403,
-        `Only \`world_readable\` rooms can be viewed in the archive. ${roomData.id} has m.room.history_visiblity=${roomData.historyVisibility}`
+        `Only \`world_readable\` rooms can be viewed in the archive. ` +
+          `${roomData.id} has m.room.history_visiblity=${roomData.historyVisibility} ` +
+          `(set by ${roomData.historyVisibilityEventMeta?.sender} on ` +
+          `${new Date(roomData.historyVisibilityEventMeta?.originServerTs).toISOString()})`
       );
     }
 

shared/hydrogen-vm-render-script.js

@@ -118,6 +118,7 @@ async function mountHydrogen() {
     events,
     stateEventMap,
     shouldIndex,
+    historyVisibilityEventMeta: roomData.historyVisibilityEventMeta,
     basePath: config.basePath,
   });
 

shared/viewmodels/ArchiveRoomViewModel.js

@@ -75,6 +75,7 @@ class ArchiveRoomViewModel extends ViewModel {
       events,
       stateEventMap,
       shouldIndex,
+      historyVisibilityEventMeta,
       basePath,
     } = options;
     assert(homeserverUrl);
@@ -85,6 +86,9 @@ class ArchiveRoomViewModel extends ViewModel {
     assert(events);
     assert(stateEventMap);
     assert(shouldIndex !== undefined);
+    assert(historyVisibilityEventMeta.historyVisibility);
+    assert(historyVisibilityEventMeta.sender);
+    assert(historyVisibilityEventMeta.originServerTs);
     assert(events);
 
     this._room = room;
@@ -213,6 +217,7 @@ class ArchiveRoomViewModel extends ViewModel {
         shouldShowTimeSelector,
         timeSelectorViewModel: this._timeSelectorViewModel,
         shouldIndex,
+        historyVisibilityEventMeta,
         get developerOptionsUrl() {
           return urlRouter.urlForSegments([
             navigation.segment('room', room.id),

shared/views/RightPanelContentView.js

@@ -10,12 +10,28 @@ class RightPanelContentView extends TemplateView {
   render(t, vm) {
     assert(vm.shouldIndex !== undefined);
     assert(vm.shouldShowTimeSelector !== undefined);
+    assert(vm.historyVisibilityEventMeta.historyVisibility);
+    assert(vm.historyVisibilityEventMeta.sender);
+    assert(vm.historyVisibilityEventMeta.originServerTs);
 
     let maybeIndexedMessage = 'This room is not being indexed by search engines ';
     if (vm.shouldIndex) {
-      maybeIndexedMessage = 'This room is being indexed by search engines ';
+      maybeIndexedMessage = 'This room is being indexed by search engines';
     }
 
+    const historyVisibilitySender = vm.historyVisibilityEventMeta.sender;
+
+    let historyVisibilityDisplayValue = vm.historyVisibilityEventMeta.historyVisibility;
+    if (vm.historyVisibilityEventMeta.historyVisibility === 'world_readable') {
+      historyVisibilityDisplayValue = 'world readable';
+    }
+
+    const [historyVisibilitySetDatePiece, _timePiece] = new Date(
+      vm.historyVisibilityEventMeta.originServerTs
+    )
+      .toISOString()
+      .split('T');
+
     return t.div(
       {
         className: 'RightPanelContentView',
@@ -33,9 +49,13 @@ class RightPanelContentView extends TemplateView {
             className: 'RightPanelContentView_footer',
           },
           [
+            t.p([
+              `This room is accessible in the archive because it was set to ` +
+                `${historyVisibilityDisplayValue} by ${historyVisibilitySender} on ${historyVisibilitySetDatePiece}.`,
+            ]),
             t.p([
               maybeIndexedMessage,
-              '(',
+              ' (',
               t.a(
                 {
                   className: 'external-link RightPanelContentView_footerLink',