QObject::timerEvent may be called before the timer elapsed

QObject::timerEvent may be called before the timer elapsed because timer ids are aggressively reused by Qt's timers. This issue can happen if

• A timer was recently stopped
• There are remaining WM_TIMER events in the Windows event queue
• QObject::startTimer is used to create a new timer

Under these conditions, QObject::startTimer will likely reuse the timer id of the previously stopped timer. This causes premature execution of QObject::timerEvent when the lingering WM_TIMER is processed.

This can for example cause premature false return from QSignalSpy even if no signals were received, which leads to flaky tests.

See also

• Why is my message queue full of WM_TIMER messages? - The Old New Thing (microsoft.com)
• Can I force a WM_TIMER message to be generated when the timer comes due, even if the message queue is not idle? - The Old New Thing (microsoft.com)