* LineVector<T> is only instantiated for Sci::Position.
  It instantiates Partitioning<T> as well and Partitioning<T>::InsertPartitions()
  is called with a Sci::Position.
  This only couldn't cause problems if ptrdiff_t aliases to int.
  In either case, I don't see why we need LineVector<int> at all.
* We need RunStyles<ptrdiff_t> as well since ptrdiff_t is long int
  and we instantiate only RunStyles<int>.
  Both types have the same storage size but are still different.
  See also this discussion about the same workaround for Haiku:
  https://groups.google.com/g/scintilla-interest/c/xPXquJUIXo8/m/BLXBpTTgBwAJ

* SciTECO needs to avoid automatic scrolling as an optimization.
  While this works partially by avoiding certain messages like SCI_GOTOPOS (in favor of SCI_SETEMPTYSELECTION),
  there aren't alternatives for all messages that scroll.
  For instance SCI_UNDO will always call EnsureCaretVisible().
  Also, even if we could avoid all scrolling messages, there is no guarantee that will not suddenly
  and unexpectedly break in the future.
* Instead, every scrolling is now disabled except for an explicit SCI_SCROLLCARET.
  SciTECO can therefore use the more intuitive messages like SCI_GOTOPOS.
* SetLastXChosen() (choosing the X that caret sticks to) has been found to be a major slow down
  during UNDO and serves no purpose on SciTECO anyway - we have to implement such an algorithm
  at the SciTECO language level.
  We therefore simply disable SetLastXChosen().

* This sets the default key representations on every SCI_SETDOCPOINTER
  which is very inefficient considering the way that SciTECO uses the
  Q-Register view.
  Furthermore it meant, we had to reset the representations to their
  SciTECO versions again after every SCI_SETDOCPOINTER.
* This patch only does not cause problems because we initialize
  the representations anyway for every Scintilla view.
  This patch is not meant to be upstreamed!

to improve performance.

identical to default initialization.
Remove constructor argument which is same as default.

Use string_view to improve safety.

Move into unnamed namespace. Don't hold onto AutoComplete*.
Simplify indexing by 2. Use const.