Merge branch 'feature/long-lines-and-font-locking'

2 files changed, 16 insertions, 1 deletions

diff --git a/doc/lispref/display.texi b/doc/lispref/display.texi
index 86f490677d1..d2e075c54ec 100644
--- a/doc/lispref/display.texi
+++ b/doc/lispref/display.texi
@@ -3480,6 +3480,12 @@ function finishes are the ones that really matter.
 
 For efficiency, we recommend writing these functions so that they
 usually assign faces to around 400 to 600 characters at each call.
+
+When the buffer text includes very long lines, these functions are
+called with the buffer narrowed to a relatively small region around
+@var{pos}, and with narrowing locked, so the functions cannot use
+@code{widen} to gain access to the rest of the buffer.
+@xref{Narrowing}.
 @end defvar
 
 @node Basic Faces
diff --git a/doc/lispref/positions.texi b/doc/lispref/positions.texi
index ca1166caac4..3a9a152f8dd 100644
--- a/doc/lispref/positions.texi
+++ b/doc/lispref/positions.texi
@@ -995,13 +995,18 @@ the entire buffer regardless of any narrowing.
 types of text, consider using an alternative facility described in
 @ref{Swapping Text}.
 
-@deffn Command narrow-to-region start end
+@deffn Command narrow-to-region start end &optional lock
 This function sets the accessible portion of the current buffer to start
 at @var{start} and end at @var{end}.  Both arguments should be character
 positions.
 
 In an interactive call, @var{start} and @var{end} are set to the bounds
 of the current region (point and the mark, with the smallest first).
+
+When @var{lock} is non-@code{nil}, calls to @code{widen}, or to
+@code{narrow-to-region} with an optional argument @var{lock}
+@code{nil}, do not produce any effect until the end of the current
+body form.
 @end deffn
 
 @deffn Command narrow-to-page &optional move-count
@@ -1027,6 +1032,10 @@ It is equivalent to the following expression:
 @end example
 @end deffn
 
+However, when @code{widen} is called inside a body form in which
+@code{narrow-to-region} was called with an optional argument
+@code{lock} non-@code{nil}, it does not produce any effect.
+
 @defun buffer-narrowed-p
 This function returns non-@code{nil} if the buffer is narrowed, and
 @code{nil} otherwise.