FIX: Optimistically fix topic timeline state issues

This is my second try at this. The first b246a63a59 raised an issue
with the event delegation not working because the topic id changed.

This adds support for delegating events to dynamic keys by passing a
function where a static key would normally be needed. This means that
each timeline will have its own unique state key and events will only
delegate to the proper topic.

app/assets/javascripts/discourse/app/components/mount-widget.js

@@ -83,6 +83,7 @@ export default Component.extend({
   afterPatch() {},
 
   eventDispatched(eventName, key, refreshArg) {
+    key = typeof key === "function" ? key(refreshArg) : key;
     const onRefresh = camelize(eventName.replace(/:/, "-"));
     this.dirtyKeys.keyDirty(key, { onRefresh, refreshArg });
     this.queueRerender();

app/assets/javascripts/discourse/app/components/topic-timeline.js

@@ -105,7 +105,10 @@ export default MountWidget.extend(Docking, {
       });
     }
 
-    this.dispatch("topic:current-post-scrolled", "timeline-scrollarea");
+    this.dispatch(
+      "topic:current-post-scrolled",
+      () => `timeline-scrollarea-${this.topic.id}`
+    );
     this.dispatch("topic:toggle-actions", "topic-admin-menu-button");
     if (!this.site.mobileView) {
       this.appEvents.on("composer:opened", this, this.queueRerender);

app/assets/javascripts/discourse/app/widgets/topic-timeline.js

@@ -2,7 +2,6 @@ import ComponentConnector from "discourse/widgets/component-connector";
 import I18n from "I18n";
 import RawHtml from "discourse/widgets/raw-html";
 import { createWidget } from "discourse/widgets/widget";
-import { deepMerge } from "discourse-common/lib/object";
 import { h } from "virtual-dom";
 import { iconNode } from "discourse-common/lib/icon-library";
 import { later } from "@ember/runloop";
@@ -76,7 +75,7 @@ function timelineDate(date) {
 
 createWidget("timeline-scroller", {
   tagName: "div.timeline-scroller",
-  buildKey: () => `timeline-scroller`,
+  buildKey: (attrs) => `timeline-scroller-${attrs.topicId}`,
 
   defaultState() {
     return { dragging: false };
@@ -144,7 +143,7 @@ createWidget("timeline-padding", {
 
 createWidget("timeline-scrollarea", {
   tagName: "div.timeline-scrollarea",
-  buildKey: () => `timeline-scrollarea`,
+  buildKey: (attrs) => `timeline-scrollarea-${attrs.topic.id}`,
 
   buildAttributes() {
     return { style: `height: ${scrollareaHeight()}px` };
@@ -239,15 +238,15 @@ createWidget("timeline-scrollarea", {
         before + SCROLLER_HEIGHT - 5 < lastReadTop || before > lastReadTop + 25;
     }
 
+    let scrollerAttrs = position;
+    scrollerAttrs.showDockedButton =
+      !attrs.mobileView && hasBackPosition && !showButton;
+    scrollerAttrs.fullScreen = attrs.fullScreen;
+    scrollerAttrs.topicId = attrs.topic.id;
+
     const result = [
       this.attach("timeline-padding", { height: before }),
-      this.attach(
-        "timeline-scroller",
-        deepMerge(position, {
-          showDockedButton: !attrs.mobileView && hasBackPosition && !showButton,
-          fullScreen: attrs.fullScreen,
-        })
-      ),
+      this.attach("timeline-scroller", scrollerAttrs),
       this.attach("timeline-padding", { height: after }),
     ];
 
@@ -410,8 +409,7 @@ createWidget("timeline-footer-controls", {
 
 export default createWidget("topic-timeline", {
   tagName: "div.topic-timeline",
-
-  buildKey: () => "topic-timeline-area",
+  buildKey: (attrs) => `topic-timeline-area-${attrs.topic.id}`,
 
   defaultState() {
     return { position: null, excerpt: null };