fix(web): React #185 crash when switching or settling threads

[LouisHaftmann]

What happened

Since updating to T3 Code Nightly 0.0.39-nightly.20260903.1267, the desktop app frequently crashes with React error #185. It primarily happens when switching between thread tabs or clicking Settle.

Diagnosis

React error #185 means “Maximum update depth exceeded.”

The server remained healthy at the reported failure time. Its trace contained successful RPC and polling activity with no corresponding server or provider failure. The desktop trace instead showed the renderer repeatedly reinitializing around the crash.

The installed nightly tag resolves to commit 5b8445b7a777ab1070aa97b062b1618971073a96, which was also current main during triage. Its latest change, fix(web): collapse the resting composer (#7855), introduced interconnected layout effects, resize observers, and parent state updates for relocated composer controls in BranchToolbar.tsx, ChatComposer.tsx, and ChatView.tsx.

The likely regression is a layout-feedback loop when the active thread changes or disappears after settlement. This attribution is strongly suggested by the timing, triggers, and source changes, but the minified stack does not identify the exact state update.

Steps to reproduce

1. Run the T3 Code desktop app on Nightly 0.0.39-nightly.20260903.1267.
2. Open multiple thread tabs.
3. Switch repeatedly between the tabs, or click Settle on an open thread.
4. Observe that the app intermittently crashes with React error Add drag-and-drop project reordering to the sidebar #185.

Version

0.0.39-nightly.20260903.1267 (5b8445b7a777ab1070aa97b062b1618971073a96)

Environment

macOS Darwin arm64 25.6.0; Node.js v24.20.0; desktop app connected to the local T3 Code server.

Evidence

T3 Code (Nightly) 0.0.39-nightly.20260903.1267
Time: 2026-09-03T08:31:14.043Z

Error: Minified React error #185
    at fi (t3code://app/assets/dist-Bx7Ty0w9.js:8:27484)
    at ui (t3code://app/assets/dist-Bx7Ty0w9.js:8:27057)
    at qo (t3code://app/assets/dist-Bx7Ty0w9.js:8:52176)
    at t3code://app/assets/dist-Bx7Ty0w9.js:8:52057
    at e.setState (t3code://app/assets/utils-gcdr7m5n.js:18:111796)
    at e.update (t3code://app/assets/utils-gcdr7m5n.js:18:111867)
    at t3code://app/assets/utils-gcdr7m5n.js:18:117181
    at Wc (t3code://app/assets/dist-Bx7Ty0w9.js:8:91922)
    at dl (t3code://app/assets/dist-Bx7Ty0w9.js:8:98385)
    at ul (t3code://app/assets/dist-Bx7Ty0w9.js:8:97487)

At the same timestamp, server RPCs and port polling completed successfully while the desktop renderer repeatedly requested local environment bootstraps and marked itself ready.

Related issues

#2087 also reported React error #185, but involved client/server clock skew after completing a turn and was fixed in April 2026. This report has different triggers and coincides with the new resting-composer layout implementation in #7855.

Fix applied or workaround

None. No newer release or commit was available during triage.

Filed by

Codex (GPT-5) via t3 triage

[Gigioxx]

Current main already contains the exact fix in #9393, merged shortly after this issue was filed. That PR reproduced the crash geometry from nightly 0.0.39-nightly.20260903.1267, removed the state-dependent layout measurement, preserved stable control keys, and added a regression test. I re-ran the focused layout and toolbar tests on current main: 91 tests pass, and the web typecheck is clean. No follow-up PR is needed; this issue can be closed as fixed by #9393.