Notepad++ bug or settngs?

Question

Suppose I type ABEF then I want to add CD after B to get the wanted text: ABCDEF.

To get the result I want I have

Answer 1

Notepad++ and many other programs support both "Insert" and "Overstrike" modes of editing. You are describing the behaviour of "Overstrike". The bottom right hand corner of the Notepad++ window should contain either "INS" or "OVR" to indicate the current mode. Pressing the "Insert" key on a normal keyboard toggles the mode. (On my keyboard the "Insert" key is in a block of six keys, the others are "Home", "End", "Delete", "Page up" and "Page down.)