(with-eval-after-load 'help-mode (progn (defun my-help-function-def--button-function (fun &optional file type) (message "my-help-function-def--button-function: %s %s %s" fun file type) (or file (setq file (find-lisp-object-file-name fun type))) (if (not file) (message "Unable to find defining file") (require 'find-func) (when (eq file 'C-source) (setq file (help-C-file-name (indirect-function fun) 'fun))) ;; Don't use find-function-noselect because it follows ;; aliases (which fails for built-in functions). (let* ((location  (find-function-search-for-symbol fun type file)) (position (cdr location))) ;;; The original behavior is to call `pop-to-buffer' commented out hereinbelow: ;;; (pop-to-buffer (car location)) ;;; ;;; The O.P. of the following thread wishes to display the function ;;; definition in the same window as the *Help* buffer: ;;; https://emacs.stackexchange.com/q/63176/2287 (set-window-buffer (selected-window) (car location)) (run-hooks 'find-function-after-hook) (if position (progn ;; Widen the buffer if necessary to go to this position. (when (or (< position (point-min)) (> position (point-max))) (widen)) (goto-char position)) (message "Unable to find location in file"))))) ;;; In Emacs 27, the default behavior is to call `help-function-def--button-function'. ;;; The O.P. of the following thread wishes to display the variable definition in ;;; the same window as the *Help* buffer: https://emacs.stackexchange.com/q/63176/2287 (define-button-type 'help-function-def :supertype 'help-xref 'help-function #'my-help-function-def--button-function 'help-echo (purecopy "mouse-2, RET: find function's definition")) (define-button-type 'help-variable-def :supertype 'help-xref 'help-function (lambda (var &optional file) (message "help-variable-def / help-function: %s %s" var file) (when (eq file 'C-source) (setq file (help-C-file-name var 'var))) (let* ((location (find-variable-noselect var file))   (position (cdr location))) ;;; The original behavior is to call `pop-to-buffer' commented out hereinbelow: ;;; (pop-to-buffer (car location)) ;;; ;;; The O.P. of the following thread wishes to display the variable ;;; definition in the same window as the *Help* buffer: ;;; https://emacs.stackexchange.com/q/63176/2287 (set-window-buffer (selected-window) (car location)) (run-hooks 'find-function-after-hook) (if position (progn ;; Widen the buffer if necessary to go to this position. (when (or (< position (point-min)) (> position (point-max))) (widen)) (goto-char position)) (message "Unable to find location in file")))) 'help-echo (purecopy "mouse-2, RET: find variable's definition")) )) 

(with-eval-after-load 'help-mode (progn (defun my-help-function-def--button-function (fun &optional file type) (message "my-help-function-def--button-function: %s %s %s" fun file type) (or file (setq file (find-lisp-object-file-name fun type))) (if (not file) (message "Unable to find defining file") (require 'find-func) (when (eq file 'C-source) (setq file (help-C-file-name (indirect-function fun) 'fun))) ;; Don't use find-function-noselect because it follows ;; aliases (which fails for built-in functions). (let* ((location (find-function-search-for-symbol fun type file)) (position (cdr location))) ;;; The original behavior is to call `pop-to-buffer' commented out hereinbelow: ;;; (pop-to-buffer (car location)) ;;; ;;; The O.P. of the following thread wishes to display the function ;;; definition in the same window as the *Help* buffer: ;;; https://emacs.stackexchange.com/q/63176/2287 (set-window-buffer (selected-window) (car location)) (run-hooks 'find-function-after-hook) (if position (progn ;; Widen the buffer if necessary to go to this position. (when (or (< position (point-min)) (> position (point-max))) (widen)) (goto-char position)) (message "Unable to find location in file"))))) ;;; In Emacs 27, the default behavior is to call `help-function-def--button-function'. ;;; The O.P. of the following thread wishes to display the function definition in ;;; the same window as the *Help* buffer: https://emacs.stackexchange.com/q/63176/2287 (define-button-type 'help-function-def :supertype 'help-xref 'help-function #'my-help-function-def--button-function 'help-echo (purecopy "mouse-2, RET: find function's definition")) (define-button-type 'help-variable-def :supertype 'help-xref 'help-function (lambda (var &optional file) (message "help-variable-def / help-function: %s %s" var file) (when (eq file 'C-source) (setq file (help-C-file-name var 'var))) (let* ((location (find-variable-noselect var file)) (position (cdr location))) ;;; The original behavior is to call `pop-to-buffer' commented out hereinbelow: ;;; (pop-to-buffer (car location)) ;;; ;;; The O.P. of the following thread wishes to display the variable ;;; definition in the same window as the *Help* buffer: ;;; https://emacs.stackexchange.com/q/63176/2287 (set-window-buffer (selected-window) (car location)) (run-hooks 'find-function-after-hook) (if position (progn ;; Widen the buffer if necessary to go to this position. (when (or (< position (point-min)) (> position (point-max))) (widen)) (goto-char position)) (message "Unable to find location in file")))) 'help-echo (purecopy "mouse-2, RET: find variable's definition")) )) 

The following answer will affect clicking on the filename link in the *Help* buffer after calling either C-h v aka M-x describe-variable or C-h f aka M-x describe-function. The answer has been tested using Emacs 27, without any user-configuration other than the code snippet hereinbelow.

A comment by the O.P. in the linked thread, which was cited by the O.P. of the current thread, states (underneath the accepted answer) as follows: "just a quick note - I made it work just changing in help-function-def and help-variable-def function calls of pop-to-buffer to switch-to-buffer ...." By examining the code at issue, I chose to use set-window-buffer. So that anyone who uses this answer in the future can see that this works, I have included some messages at the outset of the functions ... feel free to comment them out after testing it "as-is" to verify it works as advertised. Don't forget to copy the last two (2) closing parentheses at the tail end of the code snippet ... I separated them so that we can visualize that they close out the with-eval-after-load and progn statements ...

(with-eval-after-load 'help-mode (progn (defun my-help-function-def--button-function (fun &optional file type) (message "my-help-function-def--button-function: %s %s %s" fun file type) (or file (setq file (find-lisp-object-file-name fun type))) (if (not file) (message "Unable to find defining file") (require 'find-func) (when (eq file 'C-source) (setq file (help-C-file-name (indirect-function fun) 'fun))) ;; Don't use find-function-noselect because it follows ;; aliases (which fails for built-in functions). (let* ((location (find-function-search-for-symbol fun type file)) (position (cdr location))) ;;; The original behavior is to call `pop-to-buffer' commented out hereinbelow: ;;; (pop-to-buffer (car location)) ;;; ;;; The O.P. of the following thread wishes to display the variable ;;; definition in the same window as the *Help* buffer: ;;; https://emacs.stackexchange.com/q/63176/2287 (set-window-buffer (selected-window) (car location)) (run-hooks 'find-function-after-hook) (if position (progn ;; Widen the buffer if necessary to go to this position. (when (or (< position (point-min)) (> position (point-max))) (widen)) (goto-char position)) (message "Unable to find location in file"))))) ;;; In Emacs 27, the default behavior is to call `help-function-def--button-function'. ;;; The O.P. of the following thread wishes to display the variable definition in ;;; the same window as the *Help* buffer: https://emacs.stackexchange.com/q/63176/2287 (define-button-type 'help-function-def :supertype 'help-xref 'help-function #'my-help-function-def--button-function 'help-echo (purecopy "mouse-2, RET: find function's definition")) (define-button-type 'help-variable-def :supertype 'help-xref 'help-function (lambda (var &optional file) (message "help-variable-def / help-function: %s %s" var file) (when (eq file 'C-source) (setq file (help-C-file-name var 'var))) (let* ((location (find-variable-noselect var file)) (position (cdr location))) ;;; The original behavior is to call `pop-to-buffer' commented out hereinbelow: ;;; (pop-to-buffer (car location)) ;;; ;;; The O.P. of the following thread wishes to display the variable ;;; definition in the same window as the *Help* buffer: ;;; https://emacs.stackexchange.com/q/63176/2287 (set-window-buffer (selected-window) (car location)) (run-hooks 'find-function-after-hook) (if position (progn ;; Widen the buffer if necessary to go to this position. (when (or (< position (point-min)) (> position (point-max))) (widen)) (goto-char position)) (message "Unable to find location in file")))) 'help-echo (purecopy "mouse-2, RET: find variable's definition")) )) 

The following answer will affect clicking on the filename link in the *Help* buffer after calling either C-h v aka M-x describe-variable or C-h f aka M-x describe-function. The answer has been tested using Emacs 27, without any user-configuration other than the code snippet hereinbelow. It may also be possible to add an entry to the display-buffer-alist with a custom function to achieve the desired behavior, but I chose to modify the code directly instead.

A comment by the O.P. in the linked thread, which was cited by the O.P. of the current thread, states (underneath the accepted answer) as follows: "just a quick note - I made it work just changing in help-function-def and help-variable-def function calls of pop-to-buffer to switch-to-buffer ...." By examining the code at issue, I chose to use set-window-buffer. So that anyone who uses this answer in the future can see that this works, I have included some messages at the outset of the functions ... feel free to comment them out after testing it "as-is" to verify it works as advertised. Don't forget to copy the last two (2) closing parentheses at the tail end of the code snippet ... I separated them so that we can visualize that they close out the with-eval-after-load and progn statements ...

(with-eval-after-load 'help-mode (progn (defun my-help-function-def--button-function (fun &optional file type) (message "my-help-function-def--button-function: %s %s %s" fun file type) (or file (setq file (find-lisp-object-file-name fun type))) (if (not file) (message "Unable to find defining file") (require 'find-func) (when (eq file 'C-source) (setq file (help-C-file-name (indirect-function fun) 'fun))) ;; Don't use find-function-noselect because it follows ;; aliases (which fails for built-in functions). (let* ((location (find-function-search-for-symbol fun type file)) (position (cdr location))) ;;; The original behavior is to call `pop-to-buffer' commented out hereinbelow: ;;; (pop-to-buffer (car location)) ;;; ;;; The O.P. of the following thread wishes to display the function ;;; definition in the same window as the *Help* buffer: ;;; https://emacs.stackexchange.com/q/63176/2287 (set-window-buffer (selected-window) (car location)) (run-hooks 'find-function-after-hook) (if position (progn ;; Widen the buffer if necessary to go to this position. (when (or (< position (point-min)) (> position (point-max))) (widen)) (goto-char position)) (message "Unable to find location in file"))))) ;;; In Emacs 27, the default behavior is to call `help-function-def--button-function'. ;;; The O.P. of the following thread wishes to display the variable definition in ;;; the same window as the *Help* buffer: https://emacs.stackexchange.com/q/63176/2287 (define-button-type 'help-function-def :supertype 'help-xref 'help-function #'my-help-function-def--button-function 'help-echo (purecopy "mouse-2, RET: find function's definition")) (define-button-type 'help-variable-def :supertype 'help-xref 'help-function (lambda (var &optional file) (message "help-variable-def / help-function: %s %s" var file) (when (eq file 'C-source) (setq file (help-C-file-name var 'var))) (let* ((location (find-variable-noselect var file)) (position (cdr location))) ;;; The original behavior is to call `pop-to-buffer' commented out hereinbelow: ;;; (pop-to-buffer (car location)) ;;; ;;; The O.P. of the following thread wishes to display the variable ;;; definition in the same window as the *Help* buffer: ;;; https://emacs.stackexchange.com/q/63176/2287 (set-window-buffer (selected-window) (car location)) (run-hooks 'find-function-after-hook) (if position (progn ;; Widen the buffer if necessary to go to this position. (when (or (< position (point-min)) (> position (point-max))) (widen)) (goto-char position)) (message "Unable to find location in file")))) 'help-echo (purecopy "mouse-2, RET: find variable's definition")) ))