bug#17684: #22873 (multiple fake cursors); and, #17684 (crosshairs).

[Keith David Bershatsky <esq@lawlist.com>]

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VERSION: 021.002 [06/15/2019]

CHANGELOG:

- `mc_row_position':  Added tests for an empty buffer (not narrowed) and an empty
  buffer (narrowed), both of which will be treated as an AT_ZV situation if VPOS
  is the first text row in the buffer excluding the header-line-format (if any).

- Compacted all of the calls to `mc_redraw_row' so that they are one-liners.

- Miscellaneous tidying up of code; changed a few names of some struct members.


SCREENSHOTS (X11, W32, NS):

https://www.lawlist.com/images/22873_17684_light_dark_backgrounds.png


SETUP:

Step 1:  git clone -b master git://git.sv.gnu.org/emacs.git

Step 2:  In the new emacs folder, go back to an Emacs version from 04/08/2019:

git reset --hard a038df77de7b1aa2d73a6478493b8838b59e4982

Step 3:  From within the new emacs folder created in Step 1, apply the patch:

git apply /path/to/the/patch.diff

Step 4:  ./autogen.sh

Step 5:  ./configure ... [your custom options]

Step 6:  make

Step 7:  make install


USAGE:

- For a minimal working example of built-in fake cursors, type:  M-x mc-test

  ;;; TURN ON FAKE CURSORS (buffer position, cursor-type, cursor color):

  (setq mc-conf '((1 "hbar" "magenta")
                  (2 "bar" "purple")
                  (3 "box" "#00FF00")
                  (4 "hollow" "#0000FF")
                  (5 ("hbar" 3) [1.0 0.0 1.0])
                  (6 ("bar" 3) [0.0 1.0 1.0])
                  (7 "framed" "OrangeRed"))))

  ;;; TURN OFF FAKE CURSORS:

  (setq mc-conf nil)

- To try out both the crosshairs feature and the visible fill column indicator
  feature, type:  M-x +-mode

- To try out just the visible fill column indicator feature, type:  M-x fc-mode

- To try out built-in fake cursors with Magnar Sveen's multiple-cursors package,
  install that package first.  [If the multiple-cursors package is installed, an
  eval-after-load "multiple-cursors-core" statement within `crosshairs.el` will
  redefine a couple of functions and set up a few keyboard shortcuts.]  If the
  multiple-cursors package by Magnar Sveen is not already installed, then here
  are two easy ways to install that package:

  Type:  M-x mc-install

  OR, evaluate the following snippet:

  (progn
    (require 'package)
    (add-to-list 'package-archives '("melpa" . "http://melpa.milkbox.net/packages/") t)
    (package-initialize)
    (package-refresh-contents)
    (package-install 'multiple-cursors))


TODO:

- `ns_draw_window_cursor' calls `ns_clip_to_rect', which does _not_ take
  `row->clip` into consideration when drawing cursors with `NSRectFill'.  When
  it comes time to draw glyphs on top of the box/hollow family of cursors,
  `row->clip` is taken into consideration by `get_glyph_string_clip'.  Fake
  cursors can be drawn even though the glyphs cannot, resulting in hollow/box
  family of cursors without text.  The issue can be reproduced with M-x mc-test.
  [A temporary workaround is to disable `row->clip` while drawing the glyphs.]
  https://lists.gnu.org/archive/html/emacs-devel/2019-04/msg00009.html

- Add additional support for `ch_inactive_windows' and `fc_inactive_windows'
  when the variables have a `nil` value.  In such a case, the inactive windows
  should not display the crosshairs and/or visible fill column indicator.  At the
  present time, fake cursors are drawn in active/inactive windows alike.

- Deal with left/right overwritten glyphs in the w32 and X ports of Emacs.

- Is there any additional meaningful optimization that can be added to the
  three calls of `mc_pre_scroll_clean'?

- NOTE:  As to `mc_scrolling_window', it would appear that there is no tangible
  benefit to rotating the current/prospective cache of fake cursors to compare
  the data before removing the fake cursors.  When scrolling the display, only
  _some_ lines are copied to new locations.  Areas that are not overwritten may
  have fake cursors and those may not necessarily be removed if desired/current
  matrix glyphs are equal.  The test for `GLYPH_EQUAL_P' does not take into
  consideration the existence of a fake cursor, and the glyph (with a fake
  cursor) may not be updated as a result thereof.  As to lines that are not
  copied, portions may be updated and fake cursors would be removed thereby.

- When an idle-timer fires and point is at the end of a horizontally scrolled
  line in a narrow window, the temporary horizontal scroll is canceled.  Create
  an minimal working example and file a bug report.

- There is a bug affecting an older version of Emacs for the NS port that causes
  partial line flickering when the same characters are grouped together (;;;;;;)
  and MC_GLYPHLESS cursors are above or below -- having the same background color
  as the frame; e.g., black on black (used to erase a glyphless cursor).  The
  partial flickering is only noticeable with rapid fire; e.g., holding down the
  right/left arrow key.  When changing the color of the glyphless cursor, the
  issue is not present.  [@lawlist has verified that the X and HPOS coordinates
  are accurate.]

- There is a bug affecting the recorded `w->mc.lnum_pixel_width` that is
  observable when not running under gdb, but disappears when running under gdb.
  While viewing a folded org-mode buffer, the non-gdb instance had a visible
  line number pixel-width of 44, but Emacs treated it as fluctuating between
  44 and 55 as the cursor was moved to the end of line.  In the gdb instance,
  the visible line number pixel width was 55 with no fluctuation under the same
  conditions as the non-gdb instance.  This appears to be a different bug than
  bug#32177 (current line number shifts one column to the left) because the line
  number of the current line does not shift left, and changing the bidi settings
  did not correct the issue.

- The current test for `auto_hscroll_mode_p' only looks for `current_line` and
  all five related tests are based upon that assumption, which may not be true.

- Multiple Cursors:  If point is in the middle of a composite character, then
  select a fully composed character so that the fake cursor is visible.

- Implement functionality similar to the Lisp multiple-cursors by Magnar Sveen.

- Follow up with the Emacs team re bug#32177; i.e., (Current line number shifts
  one column to the left.)

- Follow up with the Emacs team re bug#32060; i.e., Horizontal Scrolling
  (Current Line):  Wrong line gets h-scrolled.

- Determine if bug #28936 needs to be fixed and help the Emacs team re same.


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