org-src.el: Use native value of `indent-tabs-mode' for indentation

* lisp/org.el (org-indent-line): Simplify native indentation inside
src block.  Ensure we add the org indentation if the line is empty.
* lisp/org-macs.el (org-do-remove-indentation): Preserve
indentation (spaces vs tabs) past the common indentation to remove.
Do not empty blank lines.
* lisp/org-src.el (org-src--contents-for-write-back): Preserve the
native indentation (spaces vs tabs).  If necessary, add a common org
indentation to the block according to org's `indent-tabs-mode'.
(org-src-font-lock-fontify-block): Display the native indentation tab
characters with a fixed width, according to the native tab width
value, to preserve vertical alignement in the org buffer.
* testing/lisp/test-org-src.el (test-org-src/indented-blocks): Update
tests.  Indentation no longer obeys `indent-tabs-mode' from the org
buffer, but is separated in an eventual org part, and the native part.

Link: https://list.orgmode.org/87a5wcez7e.fsf@localhost/T/#t

lisp/org-macs.el

@@ -402,9 +402,12 @@ line.  Return nil if it fails."
         (when skip-fl (forward-line))
 	(while (not (eobp))
 	  (let ((ind (progn (skip-chars-forward " \t") (current-column))))
-	    (cond ((eolp) (delete-region (line-beginning-position) (point)))
-		  ((< ind n) (throw :exit nil))
-		  (t (indent-line-to (- ind n))))
+	    (cond ((< ind n)
+                   (if (eolp) (delete-region (line-beginning-position) (point))
+                     (throw :exit nil)))
+		  (t (delete-region (line-beginning-position)
+                                    (progn (move-to-column n t)
+                                           (point)))))
 	    (forward-line)))
 	;; Signal success.
 	t))))

lisp/org-src.el

@@ -326,9 +326,6 @@ is 0.")
   "File name associated to Org source buffer, or nil.")
 (put 'org-src-source-file-name 'permanent-local t)
 
-(defvar-local org-src--preserve-blank-line nil)
-(put 'org-src--preserve-blank-line 'permanent-local t)
-
 (defun org-src--construct-edit-buffer-name (org-buffer-name lang)
   "Construct the buffer name for a source editing buffer.
 Format is \"*Org Src ORG-BUFFER-NAME[ LANG ]*\"."
@@ -481,12 +478,17 @@ Assume point is in the corresponding edit buffer."
                      (list (buffer-substring (point-min) eol)
                            (buffer-substring eol (point-max))))))
 	(write-back org-src--allow-write-back)
-        (preserve-blank-line org-src--preserve-blank-line)
-        marker)
+        marker indent-str)
+    ;; Compute the exact sequence of tabs and spaces used to indent up
+    ;; to `indentation-offset' in the Org buffer.
+    (setq indent-str
+          (with-temp-buffer
+            ;; Reproduce indentation parameters from org buffer.
+            (setq indent-tabs-mode use-tabs?)
+            (when (> source-tab-width 0) (setq tab-width source-tab-width))
+            (indent-to indentation-offset)
+            (buffer-string)))
     (with-current-buffer write-back-buf
-      ;; Reproduce indentation parameters from source buffer.
-      (setq indent-tabs-mode use-tabs?)
-      (when (> source-tab-width 0) (setq tab-width source-tab-width))
       ;; Apply WRITE-BACK function on edit buffer contents.
       (insert (org-no-properties (car contents)))
       (setq marker (point-marker))
@@ -496,15 +498,14 @@ Assume point is in the corresponding edit buffer."
       ;; Add INDENTATION-OFFSET to every line in buffer,
       ;; unless indentation is meant to be preserved.
       (when (> indentation-offset 0)
-	(when preserve-fl (forward-line))
+        ;; LaTeX-fragments are inline. Do not add indentation to their
+        ;; first line.
+        (when preserve-fl (forward-line))
         (while (not (eobp))
-	  (skip-chars-forward " \t")
-          (when (or (not (eolp))                               ; not a blank line
-                    (and (eq (point) (marker-position marker)) ; current line
-                         preserve-blank-line))
-	    (let ((i (current-column)))
-	      (delete-region (line-beginning-position) (point))
-	      (indent-to (+ i indentation-offset))))
+          ;; Keep empty src lines empty, even when src block is
+          ;; indented on Org side.
+          ;; See https://list.orgmode.org/725763.1632663635@apollo2.minshall.org/T/
+          (when (not (eolp)) (insert indent-str)) ; not an empty line
 	  (forward-line)))
       (set-marker marker nil))))
 
@@ -557,11 +558,6 @@ Leave point in edit buffer."
                              (org-element-parent datum) nil))
                            (t (org-current-text-indentation)))))
 	     (content-ind org-edit-src-content-indentation)
-             (blank-line (save-excursion (forward-line 0)
-                                         (looking-at-p "^[[:space:]]*$")))
-             (empty-line (and blank-line (looking-at-p "^$")))
-             (preserve-blank-line (or (and blank-line (not empty-line))
-                                      (and empty-line (= (+ block-ind content-ind) 0))))
 	     (preserve-ind
 	      (and (memq type '(example-block src-block))
 		   (or (org-element-property :preserve-indent datum)
@@ -611,7 +607,6 @@ Leave point in edit buffer."
 	(setq org-src--overlay overlay)
 	(setq org-src--allow-write-back write-back)
 	(setq org-src-source-file-name source-file-name)
-        (setq org-src--preserve-blank-line preserve-blank-line)
 	;; Start minor mode.
 	(org-src-mode)
 	;; Clear undo information so we cannot undo back to the
@@ -645,7 +640,7 @@ Leave point in edit buffer."
   "Fontify code block between START and END using LANG's syntax.
 This function is called by Emacs' automatic fontification, as long
 as `org-src-fontify-natively' is non-nil."
-  (let ((modified (buffer-modified-p)))
+  (let ((modified (buffer-modified-p)) native-tab-width)
     (remove-text-properties start end '(face nil))
     (let ((lang-mode (org-src-get-lang-mode lang)))
       (when (fboundp lang-mode)
@@ -659,6 +654,7 @@ as `org-src-fontify-natively' is non-nil."
 	      ;; Add string and a final space to ensure property change.
 	      (insert string " "))
 	    (unless (eq major-mode lang-mode) (funcall lang-mode))
+            (setq native-tab-width tab-width)
             (font-lock-ensure)
 	    (let ((pos (point-min)) next)
 	      (while (setq next (next-property-change pos))
@@ -716,6 +712,21 @@ as `org-src-fontify-natively' is non-nil."
       (when (or (facep src-face) (listp src-face))
         (font-lock-append-text-property start end 'face src-face))
       (font-lock-append-text-property start end 'face 'org-block))
+    ;; Display native tab indentation characters as spaces
+    (save-excursion
+      (goto-char start)
+      (let ((indent-offset
+	     (if org-src-preserve-indentation 0
+	       (+ (progn (backward-char)
+                         (org-current-text-indentation))
+	          org-edit-src-content-indentation))))
+        (while (re-search-forward "^[ ]*\t" end t)
+          (let* ((b (and (eq indent-offset (move-to-column indent-offset))
+                         (point)))
+                 (e (progn (skip-chars-forward "\t") (point)))
+                 (s (and b (make-string (* (- e b) native-tab-width) ? ))))
+            (when (and b (< b e)) (add-text-properties b e `(display ,s)))
+            (forward-char)))))
     ;; Clear abbreviated link folding.
     (org-fold-region start end nil 'org-link)
     (add-text-properties

lisp/org.el

@@ -19153,21 +19153,14 @@ Also align node properties according to `org-property-format'."
 		     (org-with-point-at (org-element-end element)
 		       (skip-chars-backward " \t\n")
 		       (line-beginning-position))))
-             ;; At the beginning of a blank line, do some preindentation.  This
-             ;; signals org-src--edit-element to preserve the indentation on exit
-             (when (and (looking-at-p "^[[:space:]]*$")
-                        (not org-src-preserve-indentation))
-               (let (block-content-ind some-ind)
-                 (org-with-point-at (org-element-begin element)
-                   (setq block-content-ind (+ (org-current-text-indentation)
-                                              org-edit-src-content-indentation))
-                   (forward-line)
-		   (save-match-data (re-search-forward "^[ \t]*\\S-" nil t))
-                   (backward-char)
-                   (setq some-ind (if (looking-at-p "#\\+end_src")
-                                      block-content-ind (org-current-text-indentation))))
-                 (indent-line-to (min block-content-ind some-ind))))
-	     (org-babel-do-key-sequence-in-edit-buffer (kbd "TAB")))
+             (let ((block-content-ind
+                    (when (not org-src-preserve-indentation)
+                      (org-with-point-at (org-element-property :begin element)
+                        (+ (org-current-text-indentation)
+                           org-edit-src-content-indentation)))))
+               (org-babel-do-key-sequence-in-edit-buffer (kbd "TAB"))
+               (when (and block-content-ind (looking-at-p "^$"))
+                 (indent-line-to block-content-ind))))
 	    (t
 	     (let ((column (org--get-expected-indentation element nil)))
 	       ;; Preserve current column.

testing/lisp/test-org-src.el

@@ -345,11 +345,11 @@ This is a tab:\t.
 	(insert " Foo")
 	(org-edit-src-exit)
 	(buffer-string)))))
-  ;; Global indentation obeys `indent-tabs-mode' from the original
-  ;; buffer.
-  (should
+  ;; Global indentation does not obey `indent-tabs-mode' from the
+  ;; original buffer.
+  (should-not
    (string-match-p
-    "^\t+\s*argument2"
+    "\t"
     (org-test-with-temp-text
 	"
 - Item
@@ -364,14 +364,15 @@ This is a tab:\t.
 	(org-edit-special)
 	(org-edit-src-exit)
 	(buffer-string)))))
+  ;; Tab character is preserved
   (should
    (string-match-p
-    "^\s+argument2"
+    "\targument2"
     (org-test-with-temp-text
 	"
 - Item
   #+BEGIN_SRC emacs-lisp<point>
-    (progn\n      (function argument1\n\t\targument2))
+    (progn\n      (function argument1\n    \targument2))
   #+END_SRC"
       (setq-local indent-tabs-mode nil)
       (let ((org-edit-src-content-indentation 2)
@@ -379,43 +380,59 @@ This is a tab:\t.
 	(org-edit-special)
 	(org-edit-src-exit)
 	(buffer-string)))))
-  ;; Global indentation also obeys `tab-width' from original buffer.
+  ;; Indentation does not obey `tab-width' from org buffer.
   (should
    (string-match-p
-    "^\t\\{3\\}\s\\{2\\}argument2"
+    "^  \targument2"
     (org-test-with-temp-text
 	"
-- Item
-  #+BEGIN_SRC emacs-lisp<point>
+#+BEGIN_SRC emacs-lisp
   (progn
-    (function argument1
-              argument2))
-  #+END_SRC"
+    (list argument1\n  \t<point>argument2))
+#+END_SRC"
       (setq-local indent-tabs-mode t)
       (setq-local tab-width 4)
-      (let ((org-edit-src-content-indentation 0)
+      (let ((org-edit-src-content-indentation 2)
 	    (org-src-preserve-indentation nil))
 	(org-edit-special)
+        (setq-local indent-tabs-mode t)
+        (setq-local tab-width 8)
+        (lisp-indent-line)
 	(org-edit-src-exit)
 	(buffer-string)))))
+  ;; Tab characters are displayed with `tab-width' from the native
+  ;; edit buffer.
   (should
-   (string-match-p
-    "^\t\s\\{6\\}argument2"
+   (equal
+    10
     (org-test-with-temp-text
-	"
-- Item
-  #+BEGIN_SRC emacs-lisp<point>
+     "
+#+BEGIN_SRC emacs-lisp
   (progn
-    (function argument1
-              argument2))
-  #+END_SRC"
-      (setq-local indent-tabs-mode t)
-      (setq-local tab-width 8)
-      (let ((org-edit-src-content-indentation 0)
-	    (org-src-preserve-indentation nil))
-	(org-edit-special)
-	(org-edit-src-exit)
-	(buffer-string))))))
+    (list argument1\n  \t<point>argument2))
+#+END_SRC"
+     (setq-local indent-tabs-mode t)
+     (setq-local tab-width 4)
+     (let ((org-edit-src-content-indentation 2)
+	   (org-src-preserve-indentation nil))
+       (font-lock-ensure)
+       (current-column)))))
+  ;; The initial tab characters respect org's `tab-width'.
+  (should
+   (equal
+    10
+    (org-test-with-temp-text
+     "
+#+BEGIN_SRC emacs-lisp
+\t(progn
+\t  (list argument1\n\t\t<point>argument2))
+#+END_SRC"
+     (setq-local indent-tabs-mode t)
+     (setq-local tab-width 2)
+     (let ((org-edit-src-content-indentation 2)
+	   (org-src-preserve-indentation nil))
+       (font-lock-ensure)
+       (current-column))))))
 
 (ert-deftest test-org-src/indented-latex-fragments ()
   "Test editing multiline indented LaTeX fragment."