"Ben pencere kenarında oturmayı seviyorum."

as far as I know, as long as the subject and the pronoun are the same, you dont need to repeat the personal suffix.. in this example, you already said "ben" and "yorum" in the verb so you don't need another -m in the gerund

my first guess was ben pencere kenarinda oturmak seviyorum, but sevmek fiil need accusative, so oturmak became oturmayi otherwise it would't make any sense.

another example; I want to sit beside the window = ben pencere kenarinda oturmak istiyorum.

I want YOU to sit beside the window = ben pencere kenarinda oturmaNi istiyorum.

That would make the "pencere" a specific one. There is a window in our living room, and we've talked about it a few times. And now I want to say that I love sitting by the window, referring to that specific window, then I would say "pencerenin kenarında".

But let's say we went to a restaurant. And I want to say that I like sitting by the windows. I have never seen that window before, nor do I know the scenery, but I simply like sitting by windows; literally any window would do. So no need to specify it: "pencere kenarında". Pencere is so indefinite here that, I'm sure if this was German or Dutch, it would be spelt: "het Pencerekanarı" or something. :D The "side" part is important, not "which window".

Pencere kenarı is also used in trains and busses and planes, referring to the seats located next to the window.

"alongside the window" would be something like "Pancereyle yan yana...."

"pencere eteğinde oturmayı seviyorum."

Oturmii is sometimes heard from people, but it sounds posh. Try and stick to saying Oturmayı as much as possible.