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

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

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VERSION: 022.000 [07/14/2019]


CHANGELOG:

- Fast forward from 04/08/2019 to 07/14/2019; i.e., the attached patch applies to
  a recent version of the master branch as of 07/14/2019.  Each GUI port has
  been lightly tested to verify features 17684/22873 work as expected.

- Added a new member of the `mc_contents` struct to record the `row_position`,
  which is now used (in part) by `mc_update_window_erase' to determine whether
  an AT_ZV / POST_ZV fake cursor is the same as the previous loop -- disregarding
  lack of equality of the `*.bytepos` and `*.charpos` struct members.  An example
  where this is helpful is when a user adds / removes characters from the buffer,
  but does not add / remove a row.

- As to features 17684/22873, removed all calls to CHECK_BUFFER.  None of the
  buffer lisp objects are coming from the user (outside of C), so a `BUFFERP'
  check of `w->contents` is sufficient.

- Added screen recordings of features 17684/22873.

- Miscellaneous tidying-up of code.


VIDEOS:

w32:  https://youtu.be/r3BdJVlsAnQ

ns:  https://youtu.be/bc1h8jtbXmw

x11:  https://youtu.be/aCIFhD2Xz5s


SCREENSHOTS:

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 07/14/2019:

git reset --hard ac57c5093829ee09084c562bbbc1c412179be13d

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,
  that package must be installed.  If the multiple-cursors package is already
  installed, then just (require 'crosshairs) and `crosshairs.el` will redefine a
  few of the multiple-cursors 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)
    (mc/built-in-cursors))


TODO:

- 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
  a minimal working example and file a bug report.

  (progn
    (defun test ()
    (interactive)
      (let ((ov (make-overlay (point) (1+ (point)) nil t t)))
        (overlay-put ov 'face '(:foreground "red"))))
    (global-set-key [f5] 'test)
    (split-window-horizontally)
    (switch-to-buffer (get-buffer-create "foo"))
    (setq bidi-display-reordering nil)
    (setq-local auto-hscroll-mode 'current-line)
    (dotimes (i 80)
      (insert (char-to-string (+ 65 i)))))

- `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

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

- 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.

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

- 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.]

- 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.


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