The difference between past simple and past progressive are not limited to whether things were completed or not. Any indication of completeness is heavily dependent on the actual verb and object. You can indicate incompleteness without using the progressive. Consider:
I cleaned the kitchen yesterday
You are correct that this would generally be taken to indicate completeness. However, it wouldn't necessarily mean that you have cleaned the kitchen top to bottom, spick and span, no dirt or grime anywhere. No, it means you completed whatever tasks you understand to be represented by "clean the kitchen".
You and the person you are speaking to may or may not have the same understanding of that concept, either. You might assert that you cleaned the kitchen yesterday because you wiped down all the surfaces, mopped the floor, put away all the dishes, and so on - but your mother might disagree, say that you had not properly cleaned the kitchen because you hadn't thoroughly cleaned the cooker-top.
So, then you might say:
I browsed the internet yesterday
That means that you completed the activity that you think of "browse the internet".
Replace either of those verbs with the past progressive, and you don't indicate that you didn't complete the task. Instead, you indicate that at some point that day, you were engaged in that activity. You might use it as part of a prepositional phrase indicating time:
I found that while I was cleaning the kitchen yesterday.
Or, without that 'yesterday' on the end, it might be the answer to a question:
What were you doing at 5pm yesterday?
I was browsing the internet
If you want to clearly indicate incompleteness, you have to use some extra words:
I started cleaning the kitchen yesterday
I cleaned the kitchen part-way yesterday
I cleaned the kitchen a little yesterday