Avoid looking up settings during timeline rendering (#8313)

* Avoid showHiddenEventsInTimeline lookups

* Avoid MSC3531 feature lookups

* Test that showHiddenEventsInTimeline doesn't get looked up while
rendering

* Fix code review nits

Co-authored-by: Travis Ralston <travisr@matrix.org>

test/components/structures/MessagePanel-test.js

@@ -276,6 +276,30 @@ describe('MessagePanel', function() {
             }),
         ];
     }
+
+    function mkMixedHiddenAndShownEvents() {
+        const roomId = "!room:id";
+        const userId = "@alice:example.org";
+        const ts0 = Date.now();
+
+        return [
+            TestUtilsMatrix.mkMessage({
+                event: true,
+                room: roomId,
+                user: userId,
+                ts: ts0,
+            }),
+            TestUtilsMatrix.mkEvent({
+                event: true,
+                type: "org.example.a_hidden_event",
+                room: roomId,
+                user: userId,
+                content: {},
+                ts: ts0 + 1,
+            }),
+        ];
+    }
+
     function isReadMarkerVisible(rmContainer) {
         return rmContainer && rmContainer.children.length > 0;
     }
@@ -594,6 +618,21 @@ describe('MessagePanel', function() {
         expect(els.first().prop("events").length).toEqual(5);
         expect(els.last().prop("events").length).toEqual(5);
     });
+
+    // We test this because setting lookups can be *slow*, and we don't want
+    // them to happen in this code path
+    it("doesn't lookup showHiddenEventsInTimeline while rendering", () => {
+        // We're only interested in the setting lookups that happen on every render,
+        // rather than those happening on first mount, so let's get those out of the way
+        const res = mount(<WrappedMessagePanel events={[]} />);
+
+        // Set up our spy and re-render with new events
+        const settingsSpy = jest.spyOn(SettingsStore, "getValue").mockClear();
+        res.setProps({ events: mkMixedHiddenAndShownEvents() });
+
+        expect(settingsSpy).not.toHaveBeenCalledWith("showHiddenEventsInTimeline");
+        settingsSpy.mockRestore();
+    });
 });
 
 describe("shouldFormContinuation", () => {