The "script textarea" (the one with the line numbers, as for example used in the Script Console and the scriptler plugin) has several annoying quirks (at least on Safari, didn't try on other browsers):

• doubleclick 'n drag to select multiple words does not work
• tripleclick 'n drag to select multiple lines does not work
• it insists on autoindenting lines by 2 characters, when you press return, even if it makes no sense at all
• it jumps around on every key press and mouse click (making it impossible to use the mouse at all), when you have a long line somewhere in the file, that is not currently visible.

It is usually easier to copy the text into an external editor, and paste it back after editing, than using this control.