LineNumberMargin

Adjust the the line number margin for files with more than 10000 lines

Add

events.connect(events.FILE_OPENED, function()
  local buffer = buffer
  local c = _SCINTILLA.constants
  local width = #(buffer.line_count..'')
  width = width > 4 and width or 4
  buffer.margin_width_n[1] = 4 + width * buffer:text_width(c.STYLE_LINENUMBER, '9')
end)

to your ~/.textadept/init.lua to set the line number margin to an appropriate value for large files.