rework logic, use set diff instead of !setEq to trigger clock restart

app/pages/project/instances/InstancesPage.tsx

@@ -31,7 +31,7 @@ import { CreateLink } from '~/ui/lib/CreateButton'
 import { EmptyMessage } from '~/ui/lib/EmptyMessage'
 import { PageHeader, PageTitle } from '~/ui/lib/PageHeader'
 import { TableActions } from '~/ui/lib/Table'
-import { isSetEqual } from '~/util/array'
+import { setDiff } from '~/util/array'
 import { docLinks } from '~/util/links'
 import { pb } from '~/util/path-builder'
 
@@ -108,18 +108,25 @@ export function InstancesPage() {
         // always update. we don't have to worry about doing this in all the branches below.
         transitioningInstances.current = nextTransitioning
 
-        // if no instances are transitioning, do not poll
-        if (nextTransitioning.size === 0) return false
-
-        // if the set of transitioning instances hasn't changed, we only poll if we haven't hit the timeout
-        if (isSetEqual(prevTransitioning, nextTransitioning)) {
-          const elapsed = Date.now() - pollingStartTime.current
-          return elapsed < POLL_TIMEOUT ? POLL_INTERVAL : false
+        // We use this setDiff logic instead of just checking whether
+        // the set has changed because if you have two transitioning
+        // instances and one stops transitioning, then that's a change in
+        // the set, but you shouldn't start polling because of it! What
+        // you want to look for is new transitioning instances, i.e.,
+        // nextTransitioning.difference(prevTransitioning).size > 0
+        const anyTransitioning = nextTransitioning.size > 0
+        const anyNewTransitioning = setDiff(nextTransitioning, prevTransitioning).size > 0
+
+        // if we have new transitioning instances, restart the timeout clock
+        if (anyNewTransitioning) pollingStartTime.current = Date.now()
+
+        // important that elapsed is calculated *after* bumping start time
+        const elapsed = Date.now() - pollingStartTime.current
+        if (anyTransitioning && elapsed < POLL_TIMEOUT) {
+          return POLL_INTERVAL
         }
 
-        // if we have new transitioning instances, always poll and restart the window
-        pollingStartTime.current = Date.now()
-        return POLL_INTERVAL
+        return false
       },
     }
   )

app/util/array.spec.tsx

@@ -8,7 +8,7 @@
 import { type ReactElement } from 'react'
 import { expect, test } from 'vitest'
 
-import { groupBy, intersperse, isSetEqual } from './array'
+import { groupBy, intersperse, isSetEqual, setDiff } from './array'
 
 test('groupBy', () => {
   expect(
@@ -72,3 +72,12 @@ test('isSetEqual', () => {
 
   expect(isSetEqual(new Set([{}]), new Set([{}]))).toBe(false)
 })
+
+test.only('setDiff', () => {
+  expect(setDiff(new Set(), new Set())).toEqual(new Set())
+  expect(setDiff(new Set(['a']), new Set())).toEqual(new Set(['a']))
+  expect(setDiff(new Set(), new Set(['a']))).toEqual(new Set())
+  expect(setDiff(new Set(['b', 'a', 'c']), new Set(['b', 'd']))).toEqual(
+    new Set(['a', 'c'])
+  )
+})

app/util/array.ts

@@ -48,3 +48,8 @@ export function isSetEqual<T>(a: Set<T>, b: Set<T>): boolean {
   }
   return true
 }
+
+/** Set `a - b` */
+export function setDiff<T>(a: Set<T>, b: Set<T>): Set<T> {
+  return new Set([...a].filter((x) => !b.has(x)))
+}