options: set 'scrollback' to -1 by default #9563

Makes the 'scrollback' option more consistent (same default for all buffers) and future-proof.

- Default to -1 for all buffers, but treat it as an implementation detail.
- Document range of 1 - 100_000.
- New terminal buffer by default sets scrollback=10_000 if the global default is -1.
- Existing terminal buffer: On entering terminal-mode or on refresh, if the user explicitly did `:set[local] scbk=-1`, the local value goes to 100_000 (max). (This is undocumented on purpose. Users should work with explicit values in the range of 1-100_000.)

1 files changed, 2 insertions, 4 deletions

diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index b6ea63449d..d0643bf5ed 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -4761,14 +4761,12 @@ A jump table for the options with a short description can be found at |Q_op|.
 	height with ":set scroll=0".
 
 						*'scrollback'* *'scbk'*
-'scrollback' 'scbk'	number	(default: 10000
-				 in normal buffers: -1)
+'scrollback' 'scbk'	number	(default: 10000)
 			local to buffer
 	Maximum number of lines kept beyond the visible screen. Lines at the
 	top are deleted if new lines exceed this limit.
+	Minimum is 1, maximum is 100000.
 	Only in |terminal| buffers.
-	-1 means "unlimited" for normal buffers, 100000 otherwise.
-	Minimum is 1.
 
 			*'scrollbind'* *'scb'* *'noscrollbind'* *'noscb'*
 'scrollbind' 'scb'	boolean  (default off)