bug#38966: 27.0.60; Assertion failure in set_cursor_from_row

[martin rudalics <rudalics@gmx.at>]

[-- Attachment #1: Type: text/plain, Size: 2250 bytes --]

 > I don't expect to have a window that has no lines showing text.  I
 > believe we don't allow creation/resizing of windows to such a small
 > size?  If that's not guaranteed, I'm okay with adding an assertion
 > somewhere, but that would be a separate problem: we never expected
 > such a calamity even before tab-lines were added.

We did.  Try with emacs 26 loading my old test-popup-2.el (attached
once more).  Type C-x 2 and make the lower window as low as you can.
Now hit F2.  The lower window shows no text.

So we did allow windows showing no text before and I see no reason why
we should call that a calamity.  Rather, we should eliminate all
assertions that forbid zero size windows and simply not draw a cursor
when a window's body shrinks to zero pixels.  All applications I know
of tolerate such zero size "windows".

I plan a slight modification that would allow the minibuffer window to
shrink to zero lines whenever it's not used.  OTOH, zero size "normal"
windows would allow to show the minibuffer window in a frame that can
acommodate only one or two lines.

 > If we don't call set_cursor_from_row, we will not have a cursor
 > displayed where it should be, which is a display bug in and of itself.

Isn't that statement overly conservative?  Sooner or later, Emacs will
at least have to allow a mode where the region is not destroyed when a
window is scrolled, allowing the cursor to disappear from the visible
part of its buffer.

We should keep Emacs distinctive.  But being distinctive doesn't
necessarily mean to be restrictive.

 >> BTW: I would also recommend to rename mode_line_p to something like
 >> say "no_text_line_p".  Presently, people like me not used to hack the
 >> display code, might consider mode_line_p verbatim and not apply its
 >> semantics to header or tab lines.
 >
 > Sorry, but that ship sailed a long time ago.  You will have to make do
 > with the comments in dispextern.h, which document this weirdness.

Which comment?  In either case, rest assured that ordinary people will
have considerable troubles understanding code like the proposed

	  if (row->tab_line_p)
	    ++row;
	  if (row->mode_line_p)
	    ++row;

Notwithstanding my concerns, the patch fixes the bug here.

Many thanks, martin

[-- Attachment #2: test-popup-2.el --]
[-- Type: text/plain, Size: 1705 bytes --]

(set-frame-position nil 0 0)
(switch-to-buffer (find-file-noselect "c:/temp/test-popup.el"))
(setq window-resize-pixelwise t)
(setq window-min-height 0)
(set-window-scroll-bars (minibuffer-window) 0 nil 0 nil)
(defvar old-mode-line-format mode-line-format)

(defun foo-message (&optional newline)
  (message
   (concat (if newline "\n" "")
	   "total: %s, body: %s, mode: %s")
   (window-pixel-height) (window-body-height nil t)
   (window-mode-line-height)))

(defun foo ()
  (interactive)
  (foo-message))

(global-set-key [f1] 'foo)

(defun test-popup ()
  (interactive)
;;   (set-face-attribute 'mode-line nil :height 200)
;;   (set-face-attribute 'mode-line-inactive nil :height 200)
  (setq
   mode-line-format
   (cons (propertize " " 'display
                     (create-image "/* XPM */ static char * image[] = {
\"3 60 1 1\",
\"0 c #00aaff\",
\"000\",\n\"000\",\n\"000\",\n\"000\",\n\"000\",
\"000\",\n\"000\",\n\"000\",\n\"000\",\n\"000\",
\"000\",\n\"000\",\n\"000\",\n\"000\",\n\"000\",
\"000\",\n\"000\",\n\"000\",\n\"000\",\n\"000\",
\"000\",\n\"000\",\n\"000\",\n\"000\",\n\"000\",
\"000\",\n\"000\",\n\"000\",\n\"000\",\n\"000\",
\"000\",\n\"000\",\n\"000\",\n\"000\",\n\"000\",
\"000\",\n\"000\",\n\"000\",\n\"000\",\n\"000\",
\"000\",\n\"000\",\n\"000\",\n\"000\",\n\"000\",
\"000\",\n\"000\",\n\"000\",\n\"000\",\n\"000\",
\"000\",\n\"000\",\n\"000\",\n\"000\",\n\"000\",
\"000\",\n\"000\",\n\"000\",\n\"000\",\n\"000\"
};" 'xpm t :ascent 'center))
	 mode-line-format))
  (foo-message t))

(global-set-key [f2] 'test-popup)

(defun bar ()
  (interactive)
  (setq mode-line-format old-mode-line-format))

(global-set-key [f3] 'bar)