Editor cursor can be moved to invalid positions

[westram]

cursor can be moved

1. outside the viewport
2. onto "bad position" at start of sequence

to reproduce

1. do
   • load demo.arb
   • use Sequence/InsertDeleteColumn? and insert 100 gaps at start of alignment
   • start editor (let viewport be smaller than canvas!
   • click into sequence
   • press keys END, HOME, HOME
   • if the window is not too wide, the cursor disappeared. position (in edit4-toparea) says '1'
   • by using Cursor-right, that position can be increased. EDIT4-cursor gets visible again when position 6 is reached
2. do
   • load demo.arb
   • start editor (let viewport be smaller than canvas!
   • click into sequence
   • press keys END, HOME, HOME
   • cursor is still visible now, but is placed at a "bad position" inside the spacer between infoarea and seqarea.
   • moving cursor vertically now, leaves the cursor unerased (i.e. you see multiple cursors)

Note: This has also been fixed for motif version years ago

[westram]

see also #577

[westram]

gtk-port is dead