diff --git a/packages/app/src/runtime/extension/titlebar-items.tsx b/packages/app/src/runtime/extension/titlebar-items.tsx
index 650df8ea7cb..60fbe3d86ed 100644
--- a/packages/app/src/runtime/extension/titlebar-items.tsx
+++ b/packages/app/src/runtime/extension/titlebar-items.tsx
@@ -1,4 +1,5 @@
 import { createMemo, For, Show } from "solid-js"
+import { syncSpinPhase } from "@opencode/ui/hooks"
 import { Icon } from "@opencode/ui/icon"
 import { TitlebarItem } from "@opencode/gui-extensions/sdk"
 import { useExtensionHost } from "./host"
@@ -70,7 +71,10 @@ function TitlebarItemButton(props: { item: TitlebarItem; vertical?: boolean }) {
           class="flex shrink-0 items-center justify-center"
           classList={{ "size-7": props.vertical, "size-5": !props.vertical }}
         >
-          <Show when={!props.item.busy} fallback={<span data-slot="titlebar-update-loader" aria-hidden="true" />}>
+          <Show
+            when={!props.item.busy}
+            fallback={<span ref={syncSpinPhase} data-slot="titlebar-update-loader" aria-hidden="true" />}
+          >
             <Show
               when={props.item.icon}
               fallback={
diff --git a/packages/ui/src/components/spinner.tsx b/packages/ui/src/components/spinner.tsx
index 9e950d02dc7..507f3671c2c 100644
--- a/packages/ui/src/components/spinner.tsx
+++ b/packages/ui/src/components/spinner.tsx
@@ -1,4 +1,5 @@
 import { ComponentProps, For } from "solid-js"
+import { syncSpinPhase } from "../hooks/spin-phase"
 
 const outerIndices = new Set([1, 2, 4, 7, 8, 11, 13, 14])
 
@@ -22,6 +23,7 @@ export function Spinner(props: {
   return (
     <svg
       {...props}
+      ref={syncSpinPhase}
       viewBox="0 0 15 15"
       data-component="spinner"
       classList={{
diff --git a/packages/ui/src/feedback/activity-ring/activity-ring.tsx b/packages/ui/src/feedback/activity-ring/activity-ring.tsx
index fdd0f3cf828..266988fda6b 100644
--- a/packages/ui/src/feedback/activity-ring/activity-ring.tsx
+++ b/packages/ui/src/feedback/activity-ring/activity-ring.tsx
@@ -1,4 +1,5 @@
 import { splitProps, type ComponentProps } from "solid-js"
+import { syncSpinPhase } from "../../hooks/spin-phase"
 import "./activity-ring.css"
 
 // An open arc in constant rotation, matching the iOS drawer's busy ring.
@@ -8,6 +9,7 @@ export function ActivityRing(props: ComponentProps<"svg">) {
   return (
     <svg
       {...rest}
+      ref={syncSpinPhase}
       class={local.class}
       classList={local.classList}
       width={local.width ?? 14}
diff --git a/packages/ui/src/feedback/loader/loader.tsx b/packages/ui/src/feedback/loader/loader.tsx
index 6a2d67b4942..c1be19eb47b 100644
--- a/packages/ui/src/feedback/loader/loader.tsx
+++ b/packages/ui/src/feedback/loader/loader.tsx
@@ -1,4 +1,5 @@
 import { splitProps, type ComponentProps } from "solid-js"
+import { syncSpinPhase } from "../../hooks/spin-phase"
 import "./loader.css"
 
 export function Loader(props: ComponentProps<"svg">) {
@@ -7,6 +8,7 @@ export function Loader(props: ComponentProps<"svg">) {
   return (
     <svg
       {...rest}
+      ref={syncSpinPhase}
       class={local.class}
       classList={local.classList}
       width={local.width ?? 16}
diff --git a/packages/ui/src/hooks/index.ts b/packages/ui/src/hooks/index.ts
index 8ad36c50919..d41ad2f9729 100644
--- a/packages/ui/src/hooks/index.ts
+++ b/packages/ui/src/hooks/index.ts
@@ -1,3 +1,5 @@
 export * from "./use-filtered-list"
 
 export * from "./create-auto-scroll"
+
+export * from "./spin-phase"
diff --git a/packages/ui/src/hooks/spin-phase.test.ts b/packages/ui/src/hooks/spin-phase.test.ts
new file mode 100644
index 00000000000..61f2fd10840
--- /dev/null
+++ b/packages/ui/src/hooks/spin-phase.test.ts
@@ -0,0 +1,17 @@
+import { expect, test } from "bun:test"
+import { pinSpinPhase } from "./spin-phase"
+
+test("pins only infinite loops to the timeline origin", () => {
+  const animation = (iterations: number, startTime: number) => ({
+    startTime,
+    effect: { getTiming: () => ({ iterations }) },
+  })
+
+  const loops = [animation(Infinity, 1234), animation(Infinity, 87)]
+  const fade = animation(1, 1234)
+
+  pinSpinPhase({ getAnimations: () => [...loops, fade] })
+
+  expect(loops.map((loop) => loop.startTime)).toEqual([0, 0])
+  expect(fade.startTime).toBe(1234)
+})
diff --git a/packages/ui/src/hooks/spin-phase.ts b/packages/ui/src/hooks/spin-phase.ts
new file mode 100644
index 00000000000..ee0132e2fb4
--- /dev/null
+++ b/packages/ui/src/hooks/spin-phase.ts
@@ -0,0 +1,29 @@
+import { onCleanup } from "solid-js"
+
+// ResizeObserver delivers after style and layout but before paint, so a spinner that mounts or leaves `display: none`
+// (which restarts its CSS animation) paints its first frame already in phase.
+const observer =
+  typeof ResizeObserver === "undefined"
+    ? undefined
+    : new ResizeObserver((entries) => entries.forEach((entry) => pinSpinPhase(entry.target)))
+
+/** Use as a `ref` on a looping indicator so every instance on screen shares one phase. */
+export function syncSpinPhase(el: Element) {
+  observer?.observe(el)
+  // Turning reduced motion back off restarts the animation without resizing anything.
+  el.addEventListener("animationstart", () => pinSpinPhase(el))
+  onCleanup(() => observer?.unobserve(el))
+}
+
+// Starting every infinite loop at the document timeline's origin puts all instances of one animation in phase.
+export function pinSpinPhase(el: {
+  getAnimations(
+    options: GetAnimationsOptions,
+  ): { startTime: CSSNumberish | null; effect: { getTiming(): EffectTiming } | null }[]
+}) {
+  el.getAnimations({ subtree: true })
+    .filter((animation) => animation.effect?.getTiming().iterations === Infinity && animation.startTime !== 0)
+    .forEach((animation) => {
+      animation.startTime = 0
+    })
+}