Drywall – Small seam like line of tiny bubbles patched drywall

You'll know more when you undo the screws or thumbscrews holding the fixture onto the gangbox.

• If the gangbox itself is secure, then you have some mechanical problem with the fixture or its screws or the threads on the box.
• If the gangbox is insecure and is being pulled out of the wall,
  • You can use Madison Straps (less than a dollar) to keep the box in the wall: the long part of the strap slips in next to the box and behind the wall (slide up the long end first, then slide the short down so both top and bottom are providing support), then the short straps bend over right into the box to keep the box from coming of the hole.
  • You can pull out the loose box and replace it with an old-work box, which has surface-flush flanges to keep the box from falling in, and little clamps that flip up when you tighten their screws, pulling the box tight to the wall.
• If the gangbox is insecure because it's falling into the wall, perhaps you need to firm up the crumbling hole you're trying to secure something into: new drywall from stud to stud, or some other mechanical reinforcement of the wall itself. If you're opening up the wall, use a new work gangbox that can be fastened to a stud instead of the relatively flimsy drywall/plaster.

Does it go without saying here that you should only do things you understand, and that the circuit's power should be off while you're doing this?

There's really only one right way to do this. It takes a bit of time, but it's not difficult or complicated.

You'll want to replace the tape to ensure that the seam doesn't propagate through the paint. Pick up some premixed topping compound (available in gallon or smaller buckets), and a short roll of joint tape (cheap enough to not bother experimenting with other types of paper).

Cut a length of tape just shorter than the repair, then cut 1/4" or so off one edge so it fits comfortably in the depression. Fill the depression with mud using a taping or putty knife, then lay the tape into it. Drag the knife over it to embed it in the mud.

At this point you can opt to walk away and let it dry, or try and skim over it now to save time. By letting it dry first you eliminate the risk of pulling the tape around or wrinkling it.

Skim once or twice more, dragging the knife at a low angle and pressing firmly. There should be very little sanding to do if you do this properly.