I forgot that MUA can insert newlines on long lines.

Resend the test code to the URL and attachment below.
https://gitlab.com/-/snippets/3650077

2024年2月4日(日) 5:48 Kenta USAMI <zonuexe@zonu.me>:

> I'm using Emacs 29.2 on macOS.
> GNU Emacs 29.2 (build 2, aarch64-apple-darwin23.3.0, NS appkit-2487.40
> Version 14.3 (Build 23D56))
>  of 2024-01-26
>
> This problem is most noticeable when using markdown-mode and
> goto-address-mode,
> but it also seems to occur even if goto-address-mode is disabled in
> text-mode or fundamental-mode.
>
> You can see the problem by evaluating the Lisp code below.
>
> ;;; foo.el
> (require 'ert)
> (require 'browse-url)
>
> (ert-deftest test-browse-url-url-at-point ()
>   (let* ((text "
> [![Emacs](
> https://www.gnu.org/software/emacs/images/emacs.png)](https://www.gnu.org/software/emacs/download.html
> )
> ")
>          (expected "https://www.gnu.org/software/emacs/download.html")
>          (actual (with-temp-buffer
>                    (insert text)
>                    (goto-char 76)
>                    (browse-url-url-at-point))))
>     (should (string= expected actual))))
>
> Evaluate the expression directly in the buffer or save it to a file and
> check it with the command below.
>
> $ emacs --batch -l foo.el -f ert-run-tests-batch-and-exit
>
> 2024年2月4日(日) 4:49 Eli Zaretskii <eliz@gnu.org>:
>
>> > From: Kenta USAMI <zonuexe@zonu.me>
>> > Date: Sun, 4 Feb 2024 04:17:29 +0900
>> >
>> > In the text below, move point to the second half of the URL and
>> > execute M-x browse-url-at-point.
>> >
>> > ```
>> > [![Emacs](
>> >
>> https://www.gnu.org/software/emacs/images/emacs.png)](https://www.gnu.org/software/emacs/download.html
>> > )
>> > ```
>> >
>> > Firefox and Chromium-based browsers appear to open the URL "https//
>> > www.gnu.org/software/emacs/download.html".
>> > It seems that the ":" following https is deleted, but the browser
>> actually
>> > normalizes
>> > the URL with the duplicate scheme "http://https://".
>>
>> In what version of Emacs did you see that?  And on what OS?
>>
>> Also, please post a complete recipe: do I type the above into the
>> *scratch* buffer, or into some other buffer, and what should be the
>> major-mode of that buffer?  Also, is the part inside [...] that
>> precedes the URL important for reproducing the problem?
>>
>> Thanks.
>>
>