[BUG] Claude Desktop 1.18286.0 (macOS): Cowork sidebar file preview wedges Claude Helper GPU process into persistent busy-loop (~70% CPU / ~45% GPU) — system-wide input lag; survives preview close and process kill; infinite spinner on macOS 26.5.1

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

When Claude generates a file in a Cowork session and the sidebar attempts to preview it, the preview pane stays empty with a spinning download icon that never resolves. This is format-agnostic — reproduced with .xlsx and .pdf; the spinner appears before any format-specific rendering begins, so the preview pipeline itself appears wedged.

While the spinner is up, a Claude Helper (GPU) process busy-loops:

Claude Helper (PID 792 in our capture): 67–71% CPU, 42–47% GPU, 690–722 idle wake-ups/s, 3+ min accumulated GPU time
This drives WindowServer to 58–66% CPU, producing system-wide symptoms: Finder beachballs, cursor stalls/disappearance, keyboard input lag with burst delivery — on an M4 Max that is otherwise ~87% idle
kernel_task rises to ~27% managing the load
Two secondary Claude Helper (Renderer) processes also elevate (9–20% CPU, 250–500 idle wake-ups)
Key detail: closing the preview pane does NOT release the helper. It continued busy-looping at ~66% CPU / 42% GPU for roughly 2 further minutes before finally exiting on its own, after which system responsiveness returned to normal immediately.

What Should Happen?

The sidebar preview renders the generated file (or fails fast with an error). A failed preview should never busy-loop a GPU helper process, and closing the preview pane should tear the renderer down immediately.

Error Messages/Logs

No visible error — the spinner simply never resolves. Activity Monitor captures available (process-level CPU/GPU/idle-wake-up figures above).

Steps to Reproduce

1. Fresh launch of Claude Desktop 1.18286.0 on macOS (reproduces immediately

after restart — not load-accumulation dependent)

1. In a Cowork session, ask Claude to generate a document (e.g., an Excel

worksheet) in its output container and render it in the sidebar

1. On macOS 26.5.1: sidebar preview shows an infinite spinning download icon;

file never renders. On macOS 26.5.2: file renders (xlsx shows no worksheet
structure — no rows/columns/grid), but the underlying process wedge below
still occurs

1. Open Activity Monitor: a "Claude Helper" process is pinned at ~67-78% CPU /

~36-47% GPU with ~700+ idle wake-ups/s; WindowServer climbs to ~60%+;
system-wide input lag begins (Finder beachballs, cursor stalls, keyboard
burst lag)

1. Close the preview pane: helper keeps busy-looping (on 26.5.1 it self-exited

after ~2 min; on 26.5.2 it persisted 90+ seconds with no sign of stopping)

1. Force-quit the helper in Activity Monitor: the respawned helper re-enters

the busy-loop within seconds (78.5% CPU at <5s CPU time) — wedge condition
is app-level state, not the individual process

1. Quit and relaunch Claude Desktop: everything returns to normal until the

next sidebar preview

1. Repeat with a PDF: same result (format-agnostic)

Claude Model

Not sure / Multiple models

Is this a regression?

Yes, this worked in a previous version

Last Working Version

unknown; first seen on 1.18286.0

Claude Code Version

1.18286.0 (259c3f)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

Error Messages/Logs
No visible error — the spinner simply never resolves. Activity Monitor captures available (process-level CPU/GPU/idle-wake-up figures above).

Is this a regression?
Yes — sidebar previews worked previously. Two changes coincide with onset:

Claude Desktop 1.18286.0 installed (released 2026-07-02)
macOS updated to 26.5.1 ~2 days before the failure was first noticed — timing correlates closely with onset, and the failure signature (GPU helper busy-loop + WindowServer strain) is consistent with an OS compositor/GPU-stack interaction
Retested on macOS 26.5.2 — partial fix, core bug persists:

Previews now render (both .xlsx and .pdf) — the infinite spinner is gone, so 26.5.1 appears to have triggered the visible rendering failure
However the GPU helper busy-loop persists on 26.5.2: after closing the preview pane, the Claude Helper process remained at 75–78% CPU / 36–45% GPU with ~740 idle wake-ups for 90+ seconds with no preview open (on 26.5.1 it self-exited after ~2 min; on 26.5.2 it did not)
Force-quitting the helper does not fix it: the respawned helper re-enters the busy-loop within seconds (78.5% CPU / 40.5% GPU at <5s CPU time), indicating the wedge condition is in app-level state, not the individual process
Cosmetic issue on 26.5.2: the .xlsx sidebar preview renders without any worksheet structure (no rows/columns/grid)
Related issues (same family, different surfaces/versions)
#73834 — desktop artifact-preview renderer balloons memory (renderer runaway, memory framing; this report is the CPU/GPU-spin framing)
#71425 — Cowork sidebar tabs render empty on macOS, display layer fails, data intact (v1.14271/v1.15200)
#51426 — Live Artifacts sidebar entry rollout flicker
Workaround
Avoid the sidebar preview; download and open generated files in their native apps. Once wedged, force-quitting the helper does NOT help (respawns wedged) — the only reliable reset is quitting and relaunching Claude Desktop. Painful for iterative workflows that rely on sidebar refresh to verify document edits.