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.

The change in commit 244a503d for the <Up> and <Down> keys
needs to be done for <PageUp> and <PageDown> too.

The complementary test on current_y should only be done when doing
a scroll-only, because only then the prior line can be offscreen.

This fixes https://savannah.gnu.org/bugs/?50658

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

When scrolling backward, it is not just the bottom line of the screen
that doesn't need to be redrawn: also the line /before/ the top line
doesn't need a redraw.  Mutatis mutandis for scrolling forward.

This fixes https://savannah.gnu.org/bugs/?50657.

When determining the leftedge of the current chunk, it is not simply
the leftedge that corresponds to the placewewant, but the leftedge that
corresponds to the minimum of the placewewant and the full line span.

This fixes https://savannah.gnu.org/bugs/?50653.

When typing PageUp or PageDOwn in non-smooth-scrolling mode, the cursor
should be placed at the start of the top line of the edit window.  This
means that, when the line at edittop is partially scrolled offscreen,
the cursor should be placed at openfile->firstcolumn, not at zero.

This fixes https://savannah.gnu.org/bugs/?50645

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

In do_up() when scroll_only is TRUE, if we're at the top of the screen
in softwrap mode, it's not enough to check that edittop is on fileage.
We also need to check that firstcolumn is zero.

In do_up() when scroll_only is FALSE, if we're at the top of the screen
in softwrap mode, current_y should be zero.  This is equivalent to how,
in do_down() when scroll_only is FALSE, current_y is (editwinrows - 1)
at the bottom of the screen in softwrap mode.  Since edittop can now
be partially scrolled off the screen even when it takes up the entire
screen, checking for edittop's being equal to openfile->current->next
there no longer applies.