Re: why does git set X in LESS env var?

From: Thomas Guyot <tguyot@gmail.com>
Date: Nov 3, 2023, 11:47 UTC
Message-ID: <24c46f25-6b31-437d-9f89-1e8eb74136c8@gmail.com>
In-Reply-To: <79cf1bf35ba6c9348735685b01e0f2f9@manjaro.org>

On 2023-11-02 10:19, Dragan Simic wrote:

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> On 2023-11-02 14:19, Thomas Guyot wrote:
>> That's correct, you need both and also -y0
> Hmm, I tried the following:
>
>       GIT_PAGER='less -R -F -X -c -y0'
>
> In my environment (Xfce), the result after scrolling the output of "git
> log -p" up and down a bit was about 20 copies of the same screen "page"
> in the scrollback, plus a couple of blank "pages".  Not good,
> unfortunately, and actually much worse than having just "-R -F -X".

Indeed, I did notice it with xfce4-term - I suspect different controls are used to clear the screen, with rxvt-unicode the initial one scrolls anything in the current display to the scrollback (which is important to avoid clearing the last commands/output form the scrollback), but afterward screen updates do not update the scrollback, it remains within the screen.

These options may actually affect the behavior (I have put my current values there)

     secondaryScreen: True
         Turn on/off secondary screen (default enabled).

     secondaryScroll: True
         Turn on/off secondary screen scroll (default enabled). If this
         option is enabled, scrolls on the secondary screen will change the
         scrollback buffer and, when secondaryScreen is off, switching
         to/from the secondary screen will instead scroll the screen up.

So it appears I could disable secondaryScroll to avoid getting scrolled lines into the scrollback buffer. It's also noteworthy that with secondaryScreen disabled, rxvt-unicode instead scroll things up to avoid the current screen form being wiped out.

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>> Indeed, but when less update from the bottom, it can add new lines and
>> let the overflow lines scroll up into the scrollback buffer.
>>
>> Then updating it from the top, it draws the whole page, top to bottom.
>> That's fine for a full page but not desired for a partial one. Also
>> note that on my terminal (rxvt-unicode) when less clears the screen to
>> draw the first page the current screen is rolled up into scrollback -
>> iirc that's a configurable option, it would be worth testing other
>> terminal's behavior on that. IIRC it may also erase it when using the
>> wrong termcap file.
>>
>> I haven't looked at the code, but I think it could be possibly to
>> start the -c behavior only after a full page is drawn, after exiting
>> on partial pages, which would give us the best of both worlds.
> Does the GIT_PAGER setup, as I described it above, work for you without
> the described artifacts, in any of the environments you have access to?

Yes, rxvt-unicode, with the settings mentioned above.

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>>> Just to clarify, it's the "-X" option that creates all the issues, and
>>> the "--redraw-on-quit" option is already there to replace it with no
>>> associated issues, but the trouble is that only newer versions of
>>> less(1) support the "--redraw-on-quit" option.  IOW, it's all about
>>> improving less(1) to avoid complex workarounds required to handle
>>> different versions, such as the workarounds used in bat(1).
>> TBH I haven't tested --redraw-on-quit, even on Debian Bookworm which
>> was just released a couple months ago this option isn't available. I
>> suspect that the issue isn't -X, but the scrolling behavior controlled
>> by -y and the full redraw controlled by -c.
> When you get into the terminfo entry definitions, the root cause is that
> the terminal initialization sequences contain switching to alternate
> screen, which causes screen contents to be lost when less(1) exits.
> Thus, "-X" has been actually abused in the pager setups to skip the
> terminal initialization sequences, which may also result in other
> issues.

Right - this is something that rxvt-unicode addresses with the correct settings. IIRC I disabled the secondasyScreen to be able to use the scrollback buffer (unavailable otherwise), which probably also means that with it enabled I would also have no issue without -X, but that would need testing. Also note that rxvt-unicode has its own terminfo file (xfce4-term uses xterm), and in some places I'm forced to use xterm as a fallback I get issues like current screen being wiped when switching to/from secondary screen.

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> One of the solutions is to edit the terminfo entry manually and remove
> the escape codes that cause the switching to and from alternate screen,
> which I tested, but that also introduced another issue -- the screen
> contents was always present after less(1) exited, which isn't always the
> desired behavior.

But when less scrolls down (line by line, not page by page), it always append lines and let them scroll up. Won't you see these? (page-by-page otoh will redraw the full screen without scrolling).

Also won't it wipe the *current* screen so that you won't see the commands/output you had *before* running less?

This is why rxvt-unicode has an option to disable secondary screen, and scroll contents all the way to the buffer on switch to/from secondary screen.

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>> Actually I just tested my
>> solution on xfce4-terminal and it doesn't work, the terminal still
>> push up stuff above on redraw (noteworthy is with rxvt-unicode the
>> first draw pushes the current screen contents up but no other redraw
>> does, which is what makes it work so well - I haven't tried to find
>> out what is being done exactly... OTOH the redraw on scroll down is
>> slightly noticeable there, while impossible to see on xfce4-terminal.
>> I'll install the latest less and see what happens with --redraw on
> Please test the "--redraw-on-quit" option, so far it's the best
> available solution, IMHO.

I will, not now though - need it compile less form source I guess...

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>>>> If less could only enable this behavior after the first full page
>>>> draw, that would be perfect!
>>> Could you, please, elaborate a bit on that?
>> I mentioned it slightly above, to be clear it would mean that:
>>
>> 1. less starts by just writing lined down as usual, making any lines
>> above scroll up and overflow into the scrollback buffer as usual
>> 2.  If less draws less than a page, exits as before - the effective
>> result is as if pager was cat
>> 3. If less reaches a full page and still has lines to write, it turns
>> on -c's behavior and further updates happen from the top of the
>> screen, preventing scroll up (at least on rxvt-unicode)
>>
>> Now, if all other terms misbehave here, that's an issue, making this
>> suggestion mostly useless. And considering the number of Windows users
>> we absolutely need to test Windows Terminal, and should probably test
>> MacOS's term too (whatever that is).
> Quite frankly, I think that such a solution would be like "fixing the
> fix, which is actually an abuse", as I described it above, eventually
> introducing even more issues, instead of solving the original issue.

I'll let you know my findings... I'm not convinced --redraw-on-quit is actually going to fix it for all unless this does a lot more than the option name implies (but quite happy if it does).

Regards. -- Thomas