Hi, i have an issue with this part of windows. Why the part indicated with red arrow disappears in Object Mode... and in Edit mode appears with some invisible faces.
I have duplicated this part with shift+d and flip along Y axis -1.
Thanks for help.
EDIT MODE
OBJECT MODE
I'm not sure, but you are in X-Ray mode. By scaling by -1, the normals got flipped and backfaces are a lot more transparent than the frontfaces, so they are there, but you can't see them.
Try re-calcuating the normals (you can first turn of X-Ray to see if that actually helps...).