Changing window size

When playing in windowed mode I’m not sure how to adjust window size. Sometimes double clicking the top bar let’s me switch between two given sizes, right now all it does is moving the window to the center (same as manually clicking zoom now). There’s never an arrow I could drag. What am I missing?

edit: restored default graphic options and the “zoom” works both ways again at least, even after customizing. still don’t know what triggers this to stop working and how to have it a specific size

pps: oh, moving the window when it’s big is the cause. still: is there another way to come back to the smaller window without restoring graphics? or to even chose specific sizes?