Hi. I just found another bug which is hard to describe. So I thought my picture would help me to describe the situation.
Under certain level sizes, a certain zoom and a certain resolution, I don't always see the vertical scroll bar, therefore I cannot edit the area at the bottom of the level, unless if I change zoom. My level's height is 192 pixels, and my screen resolution is 1366 x 768. I zoomed the level to x3 and discovered the error.
The area marked by the orange ring is the missing scroll bar, and the purple ring roughly marks the area I can't edit, which is approx. 16 pixels.