Notifications: fix the use of .start()/.change() animation methods

When a notification is to stop hiding (i.e., fade in), it is support to
start fading in from whatever opacity it has at the moment, not from 0.
Fixed that part to call the `.change()` method instead of `.start()` and
added a clarification comment.

When a notification is to start hiding (i.e., fade out), it is supposed
to start fading out from the maximum opacity, even if it was not fully
restored (which only happens if the cursor passed through the
notification too quickly). Left the call to `.start()` and added a
clarification comment.

Telegram/SourceFiles/window/notifications_manager_default.cpp

@@ -537,12 +537,17 @@ void Widget::hideStop() {
 	if (_hiding) {
 		_hiding = false;
 		_hidingDelayed = {};
-		_a_opacity.start([this] { opacityAnimationCallback(); }, 0., 1., st::notifyFastAnim);
+		// Using `.change` instead of `.start` because we want to start
+		// animation from the current opacity level, not blink it.
+		_a_opacity.change(1., st::notifyFastAnim);
 	}
 }
 
 void Widget::hideAnimated(float64 duration, const anim::transition &func) {
 	_hiding = true;
+	// Using `.start` instead of `.change` because we want to ensure the notification
+	// is fully restored before starting to hide it (it may not have been restored if
+	// the cursor has passed through it too quickly, see #28811).
 	_a_opacity.start([this] { opacityAnimationCallback(); }, 1., 0., duration, func);
 }