🐛 Graphical paging issues unless piped to `less`

[Iron-E]

Description

It appears that delta's paging implementation has some graphical issues. When there is preexisting text (or even just a large diff), not all of the diff will show, and scrolling up causes previous input from the terminal to be shown (and sometimes the diff will loop and repeat itself).

Expected Behavior

delta's paging behavior will work regardless of previous input, and will show the complete contents of a diff without less.

Steps to Reproduce

1. Use cat or bat to add some text to the console (what that text is, is not important— just that there is something there).
   • cat /dev/urandom works, for example. But any text will do.
2. Take some diff, using git diff | delta or git diff using the instructions in the README.

raw diff (renders poorly on GitHub)

diff --git a/README.md b/README.md index a7251d0..92cfb5e 100644 --- a/README.md +++ b/README.md @@ -83,11 +83,11 @@ function! s:FooMode() endfunction ```

-After defining said function, you can create a mapping to enter the mode. Be sure to use <expr>. Example:
+After defining said function, you can create a mapping to enter the mode. Example:

command! FooModeEnter call libmodal#Enter('FOO', funcref('s:FooMode'))
-nnoremap <expr> <leader>n FooModeEnter
+nnoremap <leader>n :FooModeEnter

• Note the funcref() call. It must be there or else libmodal#Enter won't execute properly.
  diff --git a/autoload/libmodal.vim b/autoload/libmodal.vim
  index 9b63188..a5b1aeb 100644
  --- a/autoload/libmodal.vim
  +++ b/autoload/libmodal.vim
  @@ -90,18 +90,17 @@ function! s:Get(comboDict, comboString) abort

  " Make sure the dicitonary has a key for that value.
  if has_key(a:comboDict, l:comboChar)

•   let l:valType = type(a:comboDict[l:comboChar])
  

•   let l:val = a:comboDict[l:comboChar]
  

•   let l:valType = type(l:val)
  
    if l:valType == v:t_dict
  

•   	if has_key(a:comboDict, s:EX_KEY)
  

•   		return a:comboDict
  

•   	if has_key(l:val, s:EX_KEY) && len(a:comboString) <= 1
  

•   		return l:val
    	else
  

•   		return s:Get(
  

•   		\	a:comboDict[l:comboChar], a:comboString[1:]
  

•   		\)
  

•   		return s:Get(l:val, a:comboString[1:])
    	endif
    elseif l:valType == v:t_string && len(a:comboString) <= 1
  

•   	return a:comboDict[l:comboChar]
  

•   	return l:val
    endif
  

  " The user input has run out, but there is more in the dictionary.
  elseif a:comboString == ''
  diff --git a/doc/libmodal.txt b/doc/libmodal.txt
  index 29bc787..dd45e9f 100644
  --- a/doc/libmodal.txt
  +++ b/doc/libmodal.txt
  @@ -117,11 +117,11 @@ To define a new mode, you must first create a |user-function| to pass into
  <

After defining said |user-function|, you can create a |mapping| to enter the
-mode. Be sure to use |map-|. Example:
+mode.

command! FooModeEnter call libmodal#Enter('FOO', funcref('s:FooMode'))

• nnoremap n FooModeEnter

• nnoremap n :FooModeEnter
  <

• NOTE: the |funcref()| call must be there or else |libmodal#Enter| won't

3. Scroll up and notice that all the text from step one is still visible.
   • Additionally, scroll down and notice that not all of the diff is viewable (it stops prematurely).
4. Use git diff | delta | less and attempt to reproduce step 3's behavior. None of the text from step 1 will be visible, and all of the diff will be visible.

[dandavison]

Hi @Iron-E, thanks for this. Could you look at the Mouse scrolling section of the README, which links to #58 and https://github.com/sharkdp/bat#using-a-different-pager and report on whether those solutions fix your problem?

Delta does its paging using code from bat: Both delta and bat spawn a child less process, send their output to the stdin of less, and let less do the paging for them. There are however some environment variables which they set, which can cause differences between their behavior and the behavior of a less process invoked directly by your shell. The problems you see should be present if you send the diff to bat instead of delta. But I believe they should be fixed by the advice linked above.

It's worth running something like this to double-check your values of all the relevant environment variables:

env | grep -iE '(less|bat|pager)'

[Iron-E]

Delta does its paging using code from bat…

That's interesting. bat works when its paging is activated, and it also works with git show but it isn't as feature complete with git as delta (as it mentioned in their README). I wonder what the difference could be?

---

env | grep -iE '(less|bat|pager)'

Here's what I get:

LESSHISTFILE=-
PAGER=less
LESS=-R

---

Could you look at the Mouse scrolling section of the README, which links to #58 and https://github.com/sharkdp/bat#using-a-different-pager and report on whether those solutions fix your problem?

After reading this, here's what I can interpret:

• My PAGER env variable is less, and less uses the env variable LESS uses to apply -R.

I saw from the bat README that:

If you want to enable mouse-wheel scrolling on older versions of less, you can pass just -R (as in the example above, this will disable the quit-if-one-screen feature). For less 530 or newer, it should work out of the box.

The version of less is 551 from June 2019, so it should be compatable with this feature.

Just for the sake of trying everything, I specified BAT_PAGER to less -R. That didn't seem change anything though.

---

Edit: I ran git diff | bat and git diff | bat | less just to see what would happen, and I can't reproduce this bug using bat.

[dandavison]

Just for the sake of trying everything, I specified BAT_PAGER to less -R.

Sorry, just to double-check, did you export that shell variable?

export BAT_PAGER='less -R'

I did think that should fix it. Of course you can also do cat diff | BAT_PAGER='less -R' delta.

[Iron-E]

Just for the sake of trying everything, I specified BAT_PAGER to less -R.

Sorry, just to double-check, did you export that shell variable?

export BAT_PAGER='less -R'

Yeah, I ran export— just to be sure though I ran it again by pasting it from here and it looks like I just mistyped something. It's working as expected now.

I am curious why bat worked without it, but delta required the specification. Probably isn't horribly important since its working now, but it is an oddity nonetheless.

Thanks for the help!

[dandavison]

Excellent! I'm thinking it's because of this: #181. I merged that yesterday, so it's in master, but not in a released version of delta yet.