T + 0ms

Dispatches click command on #login-btn

Dispatches synthetic mouse click event

Idle

T + 12ms

ChromeDriver returns HTTP 200 (click OK)

Executes onClick() handler; fires fetch()

Dispatches POST /api/auth/login

T + 25ms

Executes driver.findElement(By.id("balance"))

Main thread busy parsing network payload

Packet in transit over TCP/TLS

T + 28ms

Queries DOM tree for #balance node

DOM contains login form; #balance NOT mounted

Server executing database lookup

T + 30ms

FATAL: NoSuchElementException thrown! Test fails!

DOM unchanged

Server generates HTTP 200 response

T + 210ms

Test execution aborted; failure log dumped

Receives HTTP 200 response; updates state

Connection closed

T + 235ms

Process teardown

React mounts #balance into DOM (Too late!)

Idle

The Timing Problem: Browser Event Loop vs WebDriver Execution

The Microsecond Race Condition Explained