Observe size changes in tabs as a catch-all solution for incorrect underline sizing

packages/bbui/src/Tabs/Tab.svelte

@@ -1,40 +1,28 @@
 <script>
-  import { getContext, onMount, createEventDispatcher } from "svelte"
+  import { getContext, onDestroy } from "svelte"
   import Portal from "svelte-portal"
 
   export let title
   export let icon = ""
   export let id
 
-  const dispatch = createEventDispatcher()
   let selected = getContext("tab")
-  let tab_internal
-  let tabInfo
+  let observer
+  let ref
 
-  const setTabInfo = () => {
-    // If the tabs are being rendered inside a component which uses
-    // a svelte transition to enter, then this initial getBoundingClientRect
-    // will return an incorrect position.
-    // We just need to get this off the main thread to fix this, by using
-    // a 0ms timeout.
-    setTimeout(() => {
-      tabInfo = tab_internal?.getBoundingClientRect()
-      if (tabInfo && $selected.title === title) {
-        $selected.info = tabInfo
-      }
-    }, 0)
+  $: isSelected = $selected.title === title
+  $: {
+    if (isSelected && ref) {
+      observe()
+    } else {
+      stopObserving()
+    }
   }
 
-  onMount(() => {
-    setTabInfo()
-  })
-
-  //Ensure that the underline is in the correct location
-  $: {
-    if ($selected.title === title && tab_internal) {
-      if ($selected.info?.left !== tab_internal.getBoundingClientRect().left) {
-        setTabInfo()
-      }
+  const setTabInfo = () => {
+    const tabInfo = ref?.getBoundingClientRect()
+    if (tabInfo) {
+      $selected.info = tabInfo
     }
   }
 
@@ -42,10 +30,25 @@
     $selected = {
       ...$selected,
       title,
-      info: tab_internal.getBoundingClientRect(),
+      info: ref.getBoundingClientRect(),
     }
-    dispatch("click")
   }
+
+  const observe = () => {
+    if (!observer) {
+      observer = new ResizeObserver(setTabInfo)
+      observer.observe(ref)
+    }
+  }
+
+  const stopObserving = () => {
+    if (observer) {
+      observer.unobserve(ref)
+      observer = null
+    }
+  }
+
+  onDestroy(stopObserving)
 </script>
 
 <!-- svelte-ignore a11y-no-static-element-interactions -->
@@ -53,11 +56,12 @@
 <!-- svelte-ignore a11y-no-noninteractive-tabindex -->
 <div
   {id}
-  bind:this={tab_internal}
+  bind:this={ref}
   on:click={onClick}
-  class:is-selected={$selected.title === title}
+  on:click
   class="spectrum-Tabs-item"
-  class:emphasized={$selected.title === title && $selected.emphasized}
+  class:is-selected={isSelected}
+  class:emphasized={isSelected && $selected.emphasized}
   tabindex="0"
 >
   {#if icon}
@@ -72,7 +76,8 @@
   {/if}
   <span class="spectrum-Tabs-itemLabel">{title}</span>
 </div>
-{#if $selected.title === title}
+
+{#if isSelected}
   <Portal target=".spectrum-Tabs-content-{$selected.id}">
     <slot />
   </Portal>

packages/builder/src/pages/builder/app/[application]/_layout.svelte

@@ -105,10 +105,6 @@
   }
 
   onMount(async () => {
-    document.fonts.onloadingdone = e => {
-      builderStore.loadFonts(e.fontfaces)
-    }
-
     if (!hasSynced && application) {
       try {
         await API.syncApp(application)
@@ -149,19 +145,17 @@
           />
         </span>
         <Tabs {selected} size="M">
-          {#key $builderStore?.fonts}
-            {#each $layout.children as { path, title }}
-              <TourWrap stepKeys={[`builder-${title}-section`]}>
-                <Tab
-                  quiet
-                  selected={$isActive(path)}
-                  on:click={topItemNavigate(path)}
-                  title={capitalise(title)}
-                  id={`builder-${title}-tab`}
-                />
-              </TourWrap>
-            {/each}
-          {/key}
+          {#each $layout.children as { path, title }}
+            <TourWrap stepKeys={[`builder-${title}-section`]}>
+              <Tab
+                quiet
+                selected={$isActive(path)}
+                on:click={topItemNavigate(path)}
+                title={capitalise(title)}
+                id={`builder-${title}-tab`}
+              />
+            </TourWrap>
+          {/each}
         </Tabs>
       </div>
       <div class="topcenternav">

packages/builder/src/stores/builder/builder.js

@@ -14,7 +14,6 @@ export const INITIAL_BUILDER_STATE = {
   tourKey: null,
   tourStepKey: null,
   hoveredComponentId: null,
-  fonts: null,
 }
 
 export class BuilderStore extends BudiStore {
@@ -37,16 +36,6 @@ export class BuilderStore extends BudiStore {
     this.websocket
   }
 
-  loadFonts(fontFaces) {
-    const ff = fontFaces.map(
-      fontFace => `${fontFace.family}-${fontFace.weight}`
-    )
-    this.update(state => ({
-      ...state,
-      fonts: [...(state.fonts || []), ...ff],
-    }))
-  }
-
   init(app) {
     if (!app?.appId) {
       console.error("BuilderStore: No appId supplied for websocket")