Hi Nikolay
The OnHide() event is generated each time the interface is hidden with respect to PI's workspace. That means this event will be received (1) when the interface is shadowed, (2) when the user switches between workspaces, (3) when the interface is closed by clicking the x button, and since interfaces are top-level windows, (4) as a result of platform-dependent spontaneous hide events sent by the underlying operating system and/or window manager. Currently there's no way to know the actual reason that has triggered an OnHide() event. In future versions of PCL we'll have a richer set of events available to the Control and ProcessInterface classes.
I disabled the automatic memory clear feature mainly due to (1) and (2), as it is very annoying to have everything lost during a Blink session each time you want to minimize the interface, or if you have to change momentarily to a different workspace for any reason. On the other hand, the standard behavior of all static tools in PixInsight is to preserve their current settings, irrespective of whether their windows are visible or not, and Blink shouldn't be an exception. In fact, I think most experienced PixInsight users would be surprised if all selected files are lost when they open the Blink tool after a previous session.
Don't worry about memory consumption in the Blink tool. Once a set of images has been loaded, the user is responsible for their deallocation when they are no longer needed. A warning message box each time the tool is hidden would generate more annoyance than benefit, in my opinion.
