perf: calculate plaintext from HTML content in advance in rIC (#1748)

On the Nexus 5 especially, this ensures we no longer have nearly so many
"long tasks" (i.e. responsiveness is better). It moves html-to-txt
calculation to the same step as blurhash decoding, where it can be done
in requestIdleCallback (heck, maybe someday it could just be done in a
worker thread as well).

src/routes/_actions/createMakeProps.js

@@ -2,6 +2,8 @@ import { database } from '../_database/database'
 import { decode as decodeBlurhash, init as initBlurhash } from '../_utils/blurhash'
 import { mark, stop } from '../_utils/marks'
 import { get } from '../_utils/lodash-lite'
+import { statusHtmlToPlainText } from '../_utils/statusHtmlToPlainText'
+import { scheduleIdleTask } from '../_utils/scheduleIdleTask'
 
 async function getNotification (instanceName, timelineType, timelineValue, itemId) {
   return {
@@ -45,6 +47,24 @@ async function decodeAllBlurhashes (statusOrNotification) {
   }
 }
 
+async function calculatePlainTextContent (statusOrNotification) {
+  const status = statusOrNotification.status || statusOrNotification.notification.status
+  if (!status) {
+    return
+  }
+  const originalStatus = status.reblog ? status.reblog : status
+  const content = originalStatus.content || ''
+  const mentions = originalStatus.mentions || []
+  // Calculating the plaintext from the HTML is a non-trivial operation, so we might
+  // as well do it in advance, while blurhash is being decoded on the worker thread.
+  await new Promise(resolve => {
+    scheduleIdleTask(() => {
+      originalStatus.plainTextContent = statusHtmlToPlainText(content, mentions)
+      resolve()
+    })
+  })
+}
+
 export function createMakeProps (instanceName, timelineType, timelineValue) {
   let promiseChain = Promise.resolve()
 
@@ -64,7 +84,10 @@ export function createMakeProps (instanceName, timelineType, timelineValue) {
 
   async function getStatusOrNotification (itemId) {
     const statusOrNotification = await fetchFromIndexedDB(itemId)
-    await decodeAllBlurhashes(statusOrNotification)
+    await Promise.all([
+      decodeAllBlurhashes(statusOrNotification),
+      calculatePlainTextContent(statusOrNotification)
+    ])
     return statusOrNotification
   }
 

src/routes/_components/status/Status.html

@@ -129,7 +129,6 @@
   import { absoluteDateFormatter } from '../../_utils/formatters'
   import { composeNewStatusMentioning } from '../../_actions/mention'
   import { createStatusOrNotificationUuid } from '../../_utils/createStatusOrNotificationUuid'
-  import { statusHtmlToPlainText } from '../../_utils/statusHtmlToPlainText'
 
   const INPUT_TAGS = new Set(['a', 'button', 'input', 'textarea', 'label'])
   const isUserInputElement = node => INPUT_TAGS.has(node.localName)
@@ -224,8 +223,7 @@
       originalAccountId: ({ originalAccount }) => originalAccount.id,
       accountForShortcut: ({ originalAccount, notification }) => notification ? notification.account : originalAccount,
       visibility: ({ originalStatus }) => originalStatus.visibility,
-      mentions: ({ originalStatus }) => originalStatus.mentions || [],
-      plainTextContent: ({ content, mentions }) => statusHtmlToPlainText(content, mentions),
+      plainTextContent: ({ originalStatus }) => originalStatus.plainTextContent || '',
       plainTextContentOverLength: ({ plainTextContent }) => (
         // measureText() is expensive, so avoid doing it when possible.
         // Also measureText() typically only makes text shorter, not longer, so we can measure the raw length