Synonyms for Pop up:

n.

downloadable, above-the-fold, offline, always-on, multiuser, below-the-fold, online, navigable, click-and-mortar. blip, active window, checkbox, blank, bar, avatar, cell, button bar, button, box. bunt, ground out, foul, baulk, bat, balk, fly, HIT, double, catch. broiler, Barbie, cooker, blender, beater, barbecue, colander, coffee machine, coffee maker, burner. encyclopedic, fiction, bound, hardbound, loose-leaf, bookends, encyclopaedic, bookmark, bookrest. break through, shoot up, spring up, spring, erupt, come and go, emerge, descend on. pop up (noun)
crop up.

v.

appear (verb)
crop up, spring, emerge, break through.
arrive (verb)
HIT.
attend (verb)
catch.
grow (verb)
spring up.

Other synonyms:

spring up, bookrest, loose-leaf, bookends, hardbound. encyclopedic, bookmark, descend on. break through, emerge. erupt, shoot up. bound, spring. appear
break through, crop up.
Other relevant words:
erupt, spring up, HIT, shoot up, emerge.