Uhhh it's a feature that you can disable and isnt on by default... I actually use this feature all the time because it allows me to multitask but if you dont like it... instead of complaining about it just reset settings to default and you wont get this or... you can just disable the popup feature.. like samsung I'm giving you options.. it's up to you to decide which one works best for you.
Hun, that is called multi tasking. It doesnt close out right after you send a message. With pop up view you can multi-task with a lot of apps open at one time. The X is not really to small. I have giant fingers and I'm still able to click on the X. Most of the time I prefer to use pop up view especially in all of the apps I use 24/7 on a daily basis. Also if you just hit the back button (sometimes once and sometimes twice depending on what page you're on in the pop up view window,) it will go away. Also you can hit the little sqaure that has the 2 arrows pointed inward to close the app and just leave the little icon open and available for you to reopen it. Another thing, if you close the pop up window out, it does not reopen as a pop up view app when you go back and click on the app, unless you hit the sqaure with the arrows pointed inwards. For a long time I have been wishing that samsung would just make all samsung apps open up in pop up view instead and if you wanted to see it as a full screen you could just hit the button that makes the app open in full screen. If they don't do this, hopefully they will make an option for some programmed apps to open in pop up view. To me that would stand out and make samsung smart phones act more like small computers. Its not a big deal but really, there are a lot of available features and they all have functionality. You just have to learn how to use them. Obviously if you are coming from iphone, this would be something new to you, but if you have been using samsung forever, you should know how these features work. I'm glad you got it all worked out for you but you should seriously learn how to use the pop up features, it would help you benefit more from your device, especially if one day you want to run more than 2 apps at one time. It's very useful. Good luck.
If you are talking about the pop up you receive when a new text message or phone call comes in though, you can go under phone settings, messaging settings, email settings, etc and turn off the pop up notifications. It can be turned off. Check your setting out for each app that has pop up notifications turned on. Hit the 3 dot dot dot symbol at the top of the page on the phone app, messaging app, etc one by one and turn off pop up notifications. I'll take a screenshot