The selection is made by pressing the Up, Down, Left, Right keys or by simply clicking on the identifier.
The selection is not changed by pressing the PageUp, PageDown, Home, End keys, using the scrollbar, or pressing the Alt-Up, Alt-Down, Ctrl-Up, Ctrl-Down keys.
The Alt-Up and Alt-Down keys perform a search of the currently selected identifier backward or forward respectively.
The Ctrl-Up and Ctrl-Down keys scroll the disassembly text.
IDA does not highlight the segment names at the line prefix because it is not very useful.
It is possible to turn off the highlight. The appropriate checkbox is in the Options, General, Browser tab.