Give QPropertyUndoCommand::undo() its own first-time grace period

redo() has a direct-write path guarded by m_first_time: on the very
first call it writes the property immediately, and only animates on
calls after that (per setAnimated()'s documented contract). undo()
had no equivalent -- it always animated, so undo() both returned
before the property was restored (stale state visible to anything
sharing the call stack) and, with no running event loop, never
restored it at all.

The obvious fix -- reuse m_first_time in undo()'s guard too -- turns
out not to work, and I verified this with a standalone build before
picking an approach: QUndoStack::push() always calls redo() once
before any undo() can run, and redo()'s direct-write branch sets
m_first_time = true as it completes. So by the time undo() is ever
called, m_first_time has already flipped, and reusing it would make
undo() take the animate branch on every call, unconditionally --
syntactically symmetric with redo(), but behaviourally unchanged for
the exact scenario reported.

Instead, undo() gets its own m_undo_first_time flag, seeded from the
same first_time argument setAnimated() already takes, and set true by
undo()'s own direct-write branch the same way m_first_time is set by
redo()'s. That gives undo() a real, reachable direct-write path on its
own first call, independent of how many times redo() has already run.

Verified against a standalone build of just this class (as the issue's
own repro does): first redo and first undo are both now synchronous
with no event loop running; with an event loop present, both settle to
the correct value once "broken in"; behaviour for every other caller
of QPropertyUndoCommand -- everywhere that calls plain enableAnimation()
or the bare setAnimated() (first_time defaulting true) -- is provably
unchanged, since m_undo_first_time starts true either way and the
animate branch never modifies it.

Fixes #755.

sources/QPropertyUndoCommand/qpropertyundocommand.cpp

@@ -72,6 +72,7 @@ QPropertyUndoCommand::QPropertyUndoCommand(const QPropertyUndoCommand *other)
 	m_new_value     = other->m_new_value;
 	m_animate       = other->m_animate;
 	m_first_time    = other->m_first_time;
+	m_undo_first_time = other->m_undo_first_time;
 	setText(other->text());
 }
 
@@ -99,11 +100,20 @@ void QPropertyUndoCommand::enableAnimation (bool animate) {
 	@param first_time = if true,
 	the first animation is done at the first call of redo if false,
 	the first animation is done at the second call of redo.
+	The same rule applies to undo, tracked independently: redo() always
+	runs before the first undo() (QUndoStack::push() calls redo()
+	immediately), so by the time undo() can run at all, redo()'s own
+	m_first_time has already flipped true. Sharing that flag would leave
+	undo() with no instant path ever reachable in practice -- reusing it
+	is not actually symmetric, it just looks like it is. m_undo_first_time
+	gives undo() the same one-time grace period redo() has, on its own
+	first call instead of redo's.
 */
 void QPropertyUndoCommand::setAnimated(bool animate, bool first_time)
 {
 	m_animate = animate;
 	m_first_time = first_time;
+	m_undo_first_time = first_time;
 }
 
 /**
@@ -155,7 +165,7 @@ void QPropertyUndoCommand::undo()
 {
 	if (m_object->property(m_property_name) != m_old_value)
 	{
-		if (m_animate)
+		if (m_animate && m_undo_first_time)
 		{
 			QPropertyAnimation *animation = new QPropertyAnimation(m_object, m_property_name);
 			animation->setStartValue(m_new_value);
@@ -163,7 +173,10 @@ void QPropertyUndoCommand::undo()
 			animation->start(QAbstractAnimation::DeleteWhenStopped);
 		}
 		else
+		{
 			m_object->setProperty(m_property_name, m_old_value);
+			m_undo_first_time = true;
+		}
 	}
 
 	QUndoCommand::undo();

sources/QPropertyUndoCommand/qpropertyundocommand.h

@@ -58,7 +58,8 @@ class QPropertyUndoCommand : public QUndoCommand
 		const char *m_property_name;
 		QVariant m_old_value, m_new_value;
 		bool m_animate = false,
-			 m_first_time = true;
+			 m_first_time = true,
+			 m_undo_first_time = true;
 };
 
 #endif // QPROPERTYUNDOCOMMAND_H