BRIOFOX Spring Tension Curtain Rod 27-43 Inches, Rust-Resistance and Non-Slip Shower Curtain Rod for Bathroom, Gold

can rod be adjusted longer by turning when it is on the wall to make a tighter fit?

Do the shower curtain rings roll smoothly in the middle where the rod adjusts? The end caps look thick, does this cause a gap ?

Can I use this for a curtain with grommets? Does the bar open up and allow me to feed the curtain on?

What is the diameter of rod?