Improve documentation of 'header-line-indent-mode'

* doc/lispref/modes.texi (Header Lines): Rewrite the documentation
of 'header-line-indent-mode' and its two variables.  Fix the
example.
* doc/lispref/display.texi (Pixel Specification): More accurate
description of what happens with :align-to in header-lines.
Improve indexing.  (Bug#61239)

* src/buffer.c (syms_of_buffer) <header-line-format>:
* lisp/display-line-numbers.el (header-line-indent)
(header-line-indent-width, header-line-indent-mode): Doc fixes.

* etc/NEWS: Enhance the announcement of 'header-line-indent-mode'.

doc/lispref/display.texi

@@ -5285,9 +5285,10 @@ space taken by the line-number display.
   The @code{left}, @code{center}, and @code{right} positions can be
 used with @code{:align-to} to specify a position relative to the left
 edge, center, or right edge of the text area.  When the window
-displays line numbers, the @code{left} and the @code{center} positions
+displays line numbers, and @code{:align-to} is used in display
-are offset to account for the screen space taken by the line-number
+properties of buffer text (as opposed to header line, see below), the
-display.
+@code{left} and the @code{center} positions are offset to account for
+the screen space taken by the line-number display.
 
   Any of the above window elements (except @code{text}) can also be
 used with @code{:align-to} to specify that the position is relative to
@@ -5301,11 +5302,11 @@ the left-margin, use
 :align-to (+ left-margin (0.5 . left-margin))
 @end example
 
-  If no specific base offset is set for alignment, it is always relative
+  If no specific base offset is set for alignment, it is always
-to the left edge of the text area.  For example, @samp{:align-to 0} in a
+relative to the left edge of the text area.  For example,
-header-line aligns with the first text column in the text area.  When
+@samp{:align-to 0} aligns with the first text column in the text area.
-the window displays line numbers, the text is considered to start where
+When the window displays line numbers, the text is considered to start
-the space used for line-number display ends.
+where the space used for line-number display ends.
 
   A value of the form @code{(@var{num} . @var{expr})} stands for the
 product of the values of @var{num} and @var{expr}.  For example,
@@ -5317,6 +5318,35 @@ product of the values of @var{num} and @var{expr}.  For example,
 expressions.  The form @code{(- @var{expr} ...)} negates or subtracts
 the value of the expressions.
 
+@vindex header-line-format@r{, and } :align-to
+@cindex aligning header line, when line numbers are displayed
+@cindex header line alignment when line numbers are displayed
+  Text shown in the header line that uses @code{:align-to} display
+specifications is not automatically realigned when
+@code{display-line-numbers-mode} is turned on and off, or when the
+width of line numbers on display changes.  To arrange for the
+header-line text alignment to be updated, thus keeping the header-line
+text aligned with the buffer text, turn on the
+@code{header-line-indent-mode} in the buffer and use its two
+variables, @code{header-line-indent} and
+@code{header-line-indent-width}, in the display specification.
+@xref{Header Lines}.  Here's a simple example:
+
+@lisp
+(setq header-line-format
+      (concat (propertize " "
+                          'display
+                          '(space :align-to
+                                  (+ header-line-indent-width 10)))
+              "Column"))
+@end lisp
+
+@noindent
+This will keep the text @samp{Column} on the header line aligned with
+column 10 of buffer text, regardless of whether
+@code{display-line-numbers-mode} is on or off, and also when
+line-number display changes its width.
+
 @node Other Display Specs
 @subsection Other Display Specifications
 

doc/lispref/modes.texi

@@ -2578,21 +2578,70 @@ header line, for windows displaying the buffer.  The format of the value
 is the same as for @code{mode-line-format} (@pxref{Mode Line Data}).
 It is normally @code{nil}, so that ordinary buffers have no header
 line.
+@end defvar
 
-@findex header-line-indent-mode
+If @code{display-line-numbers-mode} is turned on in a buffer
-If @code{display-line-numbers-mode} is used, and you want the header
+(@pxref{Display Custom, display-line-numbers-mode,, emacs, The GNU
-line to be indented by the same amount as the buffer contents, you can
+Emacs Manual}), the buffer text is indented on display by the amount
-use the @code{header-line-indent-mode} minor mode.  This minor mode
+of screen space needed to show the line numbers.  By contrast, text of
-keeps the @code{header-line-indent} variable updated, so that you can
+the header line is not automatically indented, because a header line
-say something like:
+never displays a line number, and because the text of the header line
+is not necessarily directly related to buffer text below it.  If a
+Lisp program needs the header-line text to be aligned with buffer text
+(for example, if the buffer displays columnar data, like
+@code{tabulated-list-mode} does, @pxref{Tabulated List Mode}), it
+should turn on the minor mode @code{header-line-indent-mode}.
+
+@deffn Command header-line-indent-mode
+This buffer-local minor mode tracks the changes of the width of the
+line-number display on screen (which may vary depending on the range
+of line numbers shown in the window), and allows Lisp programs to
+arrange that header-line text is always aligned with buffer text when
+the line-number width changes.  Such Lisp programs should turn on this
+mode in the buffer, and use the variables @code{header-line-indent}
+and @code{header-line-indent-width} in the @code{header-line-format}
+to ensure it is adjusted to the text indentation at all times.
+@end deffn
+
+@defvar header-line-indent
+This variable's value is a whitespace string whose width is kept equal
+to the current width of line-numbers on display, provided that
+@code{header-line-indent-mode} is turned on in the buffer shown in the
+window.  The number of spaces is calculated under the assumption that
+the face of the header-line text uses the same font, including size,
+as the frame's default font; if that assumption is false, use
+@code{header-line-indent-width}, described below, instead.  This
+variable is intended to be used in simple situations where the
+header-line text needs to be indented as a whole to be realigned with
+buffer text, by prepending this variable's value to the actual
+header-line text.  For example, the following definition of
+@code{header-line-format}:
 
 @lisp
 (setq header-line-format
-      `("" header-line-format ,my-header-line))
+      `("" header-line-indent ,my-header-line))
 @end lisp
 
-This can be useful if you're displaying columnar data, and the header
+@noindent
-line should align with that data in the buffer.
+where @code{my-header-line} is the format string that produces the
+actual text of the header line, will make sure the header-line text
+is always indented like the buffer text below it.
+@end defvar
+
+@defvar header-line-indent-width
+This variable's value is kept updated to provide the current width, in
+units of the frame's canonical character width, used for displaying
+the line numbers, provided that @code{header-line-indent-mode} is
+turned on in the buffer shown in the window.  It can be used for
+aligning the header-line text with the buffer text when
+@code{header-line-indent} is not flexible enough.  For example, if the
+header line uses a font whose metrics is different from the default
+face's font, your Lisp program can calculate the width of line-number
+display in pixels, by multiplying the value of this variable by the
+value returned by @code{frame-char-width} (@pxref{Frame Font}), and
+then use the result to align header-line text using the
+@code{:align-to} display property spec (@pxref{Specified Space}) in
+pixels on the relevant parts of @code{header-line-frormat}.
 @end defvar
 
 @defun window-header-line-height &optional window

etc/NEWS

@@ -3946,9 +3946,11 @@ This argument can be used to override values of print-related settings.
 
 +++
 ** New minor mode 'header-line-indent-mode'.
-This is meant to be used in modes that have a header line that should
+This is meant to be used by Lisp programs that show a header line
-be kept aligned with the buffer contents when the user switches
+which should be kept aligned with the buffer contents when the user
-'display-line-numbers-mode' on or off.
+switches 'display-line-numbers-mode' on or off, and when the width of
+line-number display changes.  See the "(elisp) Header Lines" node in
+the Emacs Lisp Reference manual for more information.
 
 +++
 ** New global minor mode 'lost-selection-mode'.

lisp/display-line-numbers.el

@@ -112,19 +112,27 @@ the mode is on, set `display-line-numbers' directly."
 
 ;;;###autoload
 (defvar header-line-indent ""
-  "String to indent at the start if the header line.
+  "String of spaces to indent the beginning of header-line due to line numbers.
-This is used in `header-line-indent-mode', and buffers that have
+This is intended to be used in `header-line-format', and requires
-this switched on should have a `header-line-format' that look like:
+the `header-line-indent-mode' to be turned on, in order for the width
+of this string to be kept updated when the line-number width changes
+on display.  An example of a `header-line-format' that uses this
+variable might look like this:
 
   (\"\" header-line-indent THE-REST...)
 
+where THE-REST is the format string which produces the actual text
+of the header-line.
 Also see `header-line-indent-width'.")
 
 ;;;###autoload
 (defvar header-line-indent-width 0
-  "The width of the current line numbers displayed.
+  "The width of the current line number display in the window.
-This is updated when `header-line-indent-mode' is switched on.
+This is measured in units of the frame's canonical columns.
-
+This is updated when `header-line-indent-mode' is switched on,
+and is intended for use in `:align-to' display specifications
+that are part of `header-line-format', when portions of header-line
+text should be aligned to respective parts of buffer text.
 Also see `header-line-indent'.")
 
 (defun header-line-indent--line-number-width ()
@@ -155,21 +163,30 @@ Also see `header-line-indent'.")
 
 ;;;###autoload
 (define-minor-mode header-line-indent-mode
-  "Mode to indent the header line in `display-line-numbers-mode' buffers.
+  "Minor mode to help with alignment of header line when line numbers are shown.
-This means that the header line will be kept indented so that it
+This minor mode should be turned on in buffers which dispay header-line
-has blank space that's as wide as the displayed line numbers in
+that needs to be aligned with buffer text when `display-line-numbers-mode'
-the buffer.
+is turned on in the buffer.
 
-Buffers that have this switched on should have a
+Buffers that have this switched on should have a `header-line-format'
-`header-line-format' that look like:
+that uses the `header-line-indent' or the `header-line-indent-width'
+variables, which this mode will keep up-to-date with the current
+display of line numbers.  For example, a `header-line-format' that
+looks like this:
 
   (\"\" header-line-indent THE-REST...)
 
-The `header-line-indent-width' variable is also kept updated, and
+will make sure the text produced by THE-REST (which should be
-has the width of `header-line-format'.  This can be used, for
+a header-line format string) is always indented to be aligned on
-instance, in `:align-to' specs, like:
+display with the first column of buffer text.
 
-  (space :align-to (+ header-line-indent-width 10))"
+The `header-line-indent-width' variable is also kept updated,
+and can be used, for instance, in `:align-to' specs as part
+of `header-line-format', like this:
+
+  (space :align-to (+ header-line-indent-width 10))
+
+See also `line-number-display-width'."
   :lighter nil
   (if header-line-indent-mode
       (progn

src/buffer.c

@@ -5081,8 +5081,8 @@ the mode line appears at the bottom.  */);
 The header line appears, optionally, at the top of a window; the mode
 line appears at the bottom.
 
-Also see `header-line-indent-mode' if `display-line-number-mode' is
+Also see `header-line-indent-mode' if `display-line-numbers-mode' is
-used.  */);
+turned on and header-line text should be aligned with buffer text.  */);
 
   DEFVAR_PER_BUFFER ("mode-line-format", &BVAR (current_buffer, mode_line_format),
 		     Qnil,