When the cursor is on the first or last row of the edit window, and
thus Scroll-Down or Scroll-Up would push it offscreen, first move
the cursor away from the edge row and then scroll.

This fixes https://savannah.gnu.org/bugs/?53376.

When just scrolling and the cursor does not need to change position
(that is: it is not on the first or last row of the edit window),
then edit_scroll() has handled everything and there is no need to
additionally redraw anything or update 'placewewant'.

Also, rename a parameter and invert its logic.

Instead of keeping the cursor in the same spot on the screen,
let the cursor move with the text (whenever possible).

This makes these functions behave the same as in Vim and Emacs.

And adjust the comments accordingly.

And one for me, for the much changed keyboard stuff.

Each leading tab is converted to two tabs, and any leading four spaces
is converted to one tab.  The intended tab size (for keeping most lines
within 80 columns) is now four.

This fixes https://savannah.gnu.org/bugs/?52183

.
Tested-by: David Lawrence Ramsey <pooka109@gmail.com>
Improved-by: David Lawrence Ramsey <pooka109@gmail.com>

When we've landed on a tab and we are moving down and the tab starts
before the current chunk, then push the index forward -- otherwise we
would not advance.  When instead we're moving up and the end of the
preceding row is on the same tab as the target column AND the end of
the current row is not on that same tab, then there is some character
on this row that we can put the cursor on, so push the index forward
-- otherwise we would skip a usable row.

This fixes https://savannah.gnu.org/bugs/?52125
and fixes https://savannah.gnu.org/bugs/?52139.

This fixes http://savannah.gnu.org/bugs/?52042

.
Reported-by: Mike Frysinger <vapier@gentoo.org>

And use these constants in another context too.

When moving the cursor to the top-left corner, it is not necessary
to compute leftedge because firstcolumn IS the relevant leftedge.
Reported-by: Jordi Mallach <jordi@debian.org>

Don't skip rows that contain something when moving up, and do put
the cursor in the desired column whenever possible when moving down.

This fixes http://savannah.gnu.org/bugs/?51827
and fixes http://savannah.gnu.org/bugs/?51828.

This fixes http://savannah.gnu.org/bugs/?51821.

It's not yet good enough: when proper_x() pushes the index forward,
maybe it should also increase leftedge?  Or go_forward_chunks()?

This helps to fix https://savannah.gnu.org/bugs/?51787

.
Correction-by: David Lawrence Ramsey <pooka109@gmail.com>

This fixes https://savannah.gnu.org/bugs/?51809.

When the screen is narrower than a tab is wide, the cursor does not need
to be on the first/last line of the window before an <Up>/<Down> could
need to scroll the screen.

This fixes http://savannah.gnu.org/bugs/?51776

.
Reported-by: David Lawrence Ramsey <pooka109@gmail.com>

This is a temporary fix.  The resultant movement is erratic and
not what is desired (which is: to stay in the same screen column
as much as possible).

Also, move home to the first character after the tab if the current
chunk starts with a partial tab.

This fixes https://savannah.gnu.org/bugs/?51800

.
Original-idea-by: David Lawrence Ramsey <pooka109@gmail.com>

Using do_up() and do_end() when the user types <Left> at the start of
a line, and do_down() and do_home() when typing <Right> at line's end
can be problematic when tabs are wider than the screen, because those
functions convert indexes to columns and back again twice, thus causing
inaccuracies.  Therefore, simply adjust current and current_x directly,
and then redraw the screen.

This fixes https://savannah.gnu.org/bugs/index.php?51778.

Without them, nano still compiles for me, with everything enabled,
even when using --enable-debug, --enable-utf8, and --with-slang.

Since commit 8490f4ac, get_edge_and_target() depends on an up-to-date
current_x.  So: make sure current_x is updated when we move to the
top left of the screen to start a non-smooth PageUp or PageDown.

This fixes https://savannah.gnu.org/bugs/?51480.

Use actual_last_column() to properly adjust the cursor if placewewant
is past the end of a softwrapped chunk.

get_chunk_row() and get_chunk_leftedge() now become wrappers around
get_chunk(); the latter is only used directly in place_the_cursor()
when we need to get both the row and the leftedge.  get_chunk() now
uses the proper formula to implement varying chunk width.

Since chunk width now varies, place_the_cursor() sets the x position
relative to the leftedge in a different way that works regardless
of chunk width, update_softwrapped_line() loops until it gets all
softwrap breakpoints instead of calculating the full length in
advance and getting one full row at a time, the chunk iterators
now count leftedges instead of rows, and fix_firstcolumn() does a
full recalculation of the chunk that firstcolumn is on instead of
simply shifting it back.

Also, in update_softwrapped_line(), when a line's softwrap breakpoint
is before the last column of the edit window, a ">" is now added to
the end of it.

The workaround in place_the_cursor() for when two-column characters
straddle the edge of the screen is removed, as it's no longer needed
now that chunks end before such characters.

Furthermore, do_home() and do_end() use xplustabs() instead of
placewewant again when calculating the leftedge, since placewewant
refers to a column that may or may not be available (if it's not,
the cursor will be placed wrongly).  Make get_edge_and_target() use
xplustabs() instead of placewewant for the same reason; this also lets
us simplify get_edge_and_target(), since xplustabs() will never be
greater than strlenpt().  Finally, since do_end() now has to calculate
rightedge as well as rightedge_x, use the former to implement the same
time-saving optimizations as in do_home().

The cursor is not yet adjusted when we try to go directly to a column
past the end of a softwrap breakpoint, and placewewant handling in the
vertical movement code is not yet adjusted for varying chunk lengths,
but fixes for these are forthcoming.

This fixes https://savannah.gnu.org/bugs/?49440.

get_chunk_row() replaces the formula "column / editwincols".

get_chunk_leftedge() replaces "(column / editwincols) * editwincols".

get_last_chunk_row() replaces "strlenpt() / editwincols".

get_last_chunk_leftedge() replaces "(strlenpt() / editwincols) * editwincols".

This prepares us for any changes in those formulas, and for more such
functions later.

The interval 2013-2017 for the Free Software Foundation is valid
because in those years there were releases with changes by either
Chris or David, and the GNU maintainers guide advises to mention
a new year in all files of a package, not just in the ones that
actually changed, and be done with it for the rest of the year.

To make dynamic Home and End work properly when double-width characters
straddle a chunk boundary, use the spot where the cursor is really shown
instead of the "actual x" position of the current character, because the
latter might be on the preceding row.

This fixes https://savannah.gnu.org/bugs/?50737.