Cursor at the wrong end of selection after deselecting

Description of the problem or steps to reproduce

Have two lines of text one below the other, select some text from left to right on one line, then press up/down arrow key.

Eg:

• 
  Press down arrow key here
• 
  It ends up here

You would expect the cursor to stay in the rightmost position, but it gets reset to the leftmost one instead.

Proposed solution

in the Deselect method, there's currently a start bool property, which decides whether to reset cursor location to the start or end of selection. But in these cases, the cursor.Loc attribute is already pointing at the correct position, so we don't need to reset c.Loc at all.

So instead of a boolean, we can make 3 possible values for it: ResetLocationStart, ResetLocationEnd, and ResetLocationDefault.

Specifications

Version: 2.0.8
Commit hash: cfcb2e45
OS: Linux
Terminal: GNOME Terminal

Locate the Deselect method and trace its callers to understand how selection reset and cursor.Loc interact with up/down movement. Reproduce the two-line selection case, then verify that deselecting preserves the cursor's correct end position without regressing cases that reset to the start or end.