An example of a text editor that has got this behaviour right IMO is HippoEdit:
1) Using Left/Right Cursor arrows or clicking with left mouse on editor area will set the horizontal insertion positon (eg. character 50). Inserting a character, pasting text or using up/down cursor keys does not change horiz insertion position.
2) Pressing Down cursor on last line moves cursor to end, but as per (1) does not change horiz insertion position. Pressing Up on first line moves cursor to start, but as per (1) does not change horiz insertion position. eg. If cursor is on first line and I press right cursor key the cursor and horiz insertion position changes to character 20. Pressing Up cursor moves cursor to start, but pressing down moves cursor to line 2 and horiz position 20 or end of line (whichever is less). Pressing Up cursor move curor back to first line and it’s also on horiz position 20.
Above behaviour (1) is useful for navigation so you don’t have to move fingers off cursor keys, and some laptops don’t include Home/End keys as well. Aboive behaviour (2) is useful as you sometimes want to inser different text at same horizontal position on different lines. It allows you to simply type/paste, move cursor up/down and type/paste again, repeat as many times as you need.
Once you have above behaviour, it makes it frustrating to use an editor without it.
Please consider implementing above behaviour in EmEditor.
I’m interested in what other people think as well.