Add an event-loop responsiveness watchdog (discussion #644 follow-up)

QetLogger (discussion #644, steps 1-3) captures whatever an explicit
qDebug()/qInfo()/qWarning() call already decided to report. Most of a
session -- painting, dragging, a slow synchronous operation -- produces
no log output at all, so a silent multi-second gap in the log is
indistinguishable from the user simply not doing anything. That gap
came up directly: investigating a user-reported "the program lagged"
required inferring stalls from timestamp gaps between unrelated log
lines, which can't tell a real freeze apart from normal idle time.

EventLoopWatchdog closes that gap directly instead of inferring it. A
QTimer::PreciseTimer repeating tick (every 50ms) measures the *actual*
elapsed time since the previous tick via QElapsedTimer (monotonic,
unaffected by system clock/NTP adjustments). Qt does not queue up
missed fires for a normal repeating timer, so if the main thread is
blocked for 600ms, the timer fires once as soon as the loop frees up,
with ~600ms measured since the last tick -- that gap is the stall,
measured at its source. Only logs (via the existing qWarning() path,
so it reuses QetLogger's file/ring/rotation with no new plumbing) when
a tick is late by more than 200ms, so a healthy session produces zero
output from this class, in keeping with QetLogger's bounded-log design.

Same QET_WATCHDOG_DISABLE=1 escape-hatch convention as QetLogger's own
QET_LOG_DISABLE=1.

Deliberately not included: attributing a stall to what caused it. This
tells you a stall happened and how long -- pairing that timestamp with
gdb attached to a running session (as used for the CLI hang, PR #661)
is still how you get from "it stalled" to a root cause.

Stacked on #647 (feature-diagnostic-logging-crash) for QetLogger/
qWarning() plumbing this depends on -- diff includes its commits until
that merges.

Verified against the compiled binary, not just read: temporarily
injected a QThread::msleep(600) via a one-shot QTimer 2s after
startup, confirmed the exact expected warning
("EventLoopWatchdog: main thread stalled for 620 ms") at the right
severity through the real qWarning()/QetLogger path, then removed the
test hook and reconfirmed a normal run produces no output from this
class at all.

cmake/qet_compilation_vars.cmake

@@ -118,6 +118,8 @@ set(QET_SRC_FILES
   ${QET_DIR}/sources/cli_export.h
   ${QET_DIR}/sources/logging/crashhandler.cpp
   ${QET_DIR}/sources/logging/crashhandler.h
+  ${QET_DIR}/sources/logging/eventloopwatchdog.cpp
+  ${QET_DIR}/sources/logging/eventloopwatchdog.h
   ${QET_DIR}/sources/logging/logring.cpp
   ${QET_DIR}/sources/logging/logring.h
   ${QET_DIR}/sources/logging/qetlogger.cpp

sources/logging/eventloopwatchdog.cpp

@@ -0,0 +1,58 @@
+/*
+	Copyright 2006-2026 The QElectroTech Team
+	This file is part of QElectroTech.
+
+	QElectroTech is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 2 of the License, or
+	(at your option) any later version.
+
+	QElectroTech is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with QElectroTech.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#include "eventloopwatchdog.h"
+
+#include <QDebug>
+#include <QProcessEnvironment>
+
+EventLoopWatchdog::EventLoopWatchdog(QObject *parent) :
+	QObject(parent)
+{
+	m_disabled = QProcessEnvironment::systemEnvironment()
+			.value(QStringLiteral("QET_WATCHDOG_DISABLE")) == QStringLiteral("1");
+
+	// Precise, not the default Coarse: Coarse explicitly trades timing
+	// accuracy for power/scheduling efficiency (platform-dependent, but
+	// commonly +/- a double-digit percentage), which would show up as
+	// noise indistinguishable from a real stall in exactly the
+	// measurement this class exists to make trustworthy.
+	m_timer.setTimerType(Qt::PreciseTimer);
+	connect(&m_timer, &QTimer::timeout, this, &EventLoopWatchdog::tick);
+}
+
+void EventLoopWatchdog::start()
+{
+	if (m_disabled)
+		return;
+
+	m_elapsed.start();
+	m_timer.start(kTickIntervalMs);
+}
+
+void EventLoopWatchdog::tick()
+{
+	// restart() returns the elapsed time and resets the clock in one
+	// call, so this tick's own cost is never counted against the next.
+	const qint64 actual_ms = m_elapsed.restart();
+
+	if (actual_ms > kStallThresholdMs) {
+		qWarning() << "EventLoopWatchdog: main thread stalled for"
+			   << actual_ms << "ms (expected a tick every"
+			   << kTickIntervalMs << "ms)";
+	}
+}

sources/logging/eventloopwatchdog.h

@@ -0,0 +1,92 @@
+/*
+	Copyright 2006-2026 The QElectroTech Team
+	This file is part of QElectroTech.
+
+	QElectroTech is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 2 of the License, or
+	(at your option) any later version.
+
+	QElectroTech is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with QElectroTech.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#ifndef EVENTLOOPWATCHDOG_H
+#define EVENTLOOPWATCHDOG_H
+
+#include <QElapsedTimer>
+#include <QObject>
+#include <QTimer>
+
+/**
+	@brief The EventLoopWatchdog class
+	Detects when the main (GUI) thread's event loop goes unresponsive --
+	QetLogger (discussion #644) can only see what an explicit qDebug()/
+	qInfo()/qWarning() call already decided to report, and most of a
+	session (painting, dragging, a slow synchronous operation) produces
+	no log output at all, so a silent multi-second gap in the log is
+	indistinguishable from the user simply not doing anything.
+
+	This closes that gap the direct way: a repeating QTimer::PreciseTimer
+	ticks on a short, fixed interval; each tick measures the *actual*
+	wall-clock time elapsed since the previous one via QElapsedTimer
+	(monotonic -- unaffected by system clock/NTP adjustments, unlike
+	QDateTime). Qt does not queue up missed fires for a normal repeating
+	timer, so if the event loop is blocked for 600ms, the timer fires
+	once as soon as the loop frees up, with ~600ms measured since the
+	last tick -- that gap *is* the stall, measured at its source rather
+	than inferred from log silence.
+
+	Only fires a qWarning() (and so only touches the log at all) when a
+	tick is late by more than kStallThresholdMs, to stay within the
+	spirit of QetLogger's bounded-log design (see its class comment) --
+	a healthy session should produce zero output from this class. This
+	tells you *that* a stall happened and *how long* it was, not what
+	caused it; pair a reported timestamp with `docker exec`+gdb the way
+	the CLI hang (PR #661) was diagnosed to go from "it lagged" to a
+	root cause.
+
+	Escape hatch: if QET_WATCHDOG_DISABLE=1 is set in the environment at
+	construction time, start() does nothing.
+*/
+class EventLoopWatchdog : public QObject
+{
+		Q_OBJECT
+
+	public:
+		/// How often the watchdog checks in. Small enough to bound the
+		/// measurement's own granularity, large enough that the tick
+		/// itself is negligible overhead on the event loop it's watching.
+		static constexpr int kTickIntervalMs = 50;
+
+		/// A tick arriving later than this many ms after the previous one
+		/// is logged as a stall. Comfortably above kTickIntervalMs so
+		/// ordinary OS scheduling noise doesn't produce a warning on every
+		/// tick, and in the range a user would actually notice as lag.
+		static constexpr int kStallThresholdMs = 200;
+
+		explicit EventLoopWatchdog(QObject *parent = nullptr);
+
+		/// Starts ticking. Must be called from the main thread, after the
+		/// event loop it watches is about to run (i.e. immediately before
+		/// QApplication::exec()) -- constructing this class earlier is
+		/// harmless, but start() before there is an event loop to tick
+		/// against would just measure the time until app.exec() is
+		/// reached. No-op if QET_WATCHDOG_DISABLE=1 was set at
+		/// construction time.
+		void start();
+
+	private slots:
+		void tick();
+
+	private:
+		QTimer m_timer;
+		QElapsedTimer m_elapsed;
+		bool m_disabled = false;
+};
+
+#endif // EVENTLOOPWATCHDOG_H

sources/main.cpp

@@ -16,6 +16,7 @@
 	along with QElectroTech.  If not, see <http://www.gnu.org/licenses/>.
 */
 #include "cli_export.h"
+#include "logging/eventloopwatchdog.h"
 #include "logging/qetlogger.h"
 #include "machine_info.h"
 #include "qet.h"
@@ -209,6 +210,13 @@ QGuiApplication::setHighDpiScaleFactorRoundingPolicy(QetSettings::hdpiScaleFacto
 		QetLogger::instance().pruneOldLogFiles(7);
 		MachineInfo::instance()->send_info_to_debug();
 	});
+
+	// Constructed here rather than earlier: start() measures ticks against
+	// the event loop app.exec() is about to run, so there is no point
+	// (and no accurate baseline) before this line.
+	EventLoopWatchdog watchdog;
+	watchdog.start();
+
 	return app.exec();
 }