RE: Box attribute in mode-line and scrolling

["Drew Adams" <drew.adams@oracle.com>]

>> >> There seems to be a conflict between scrolling and the box 
>> >> attribute in mode-line. I have in my config a ":box t"
>> >> attribute in mode-line (from the theme I use, zenburn.el).
>> >> With this setting on, C-v M-v produces a net displacement
>> >> of one line up, which is annoying. This seems to be
>> >> irrespective of settings such as scroll-conservatively or
>> >> scroll-preserve-screen-position.
>> >> 
>> >> This is a pretty old bug. It might (or might not) be
>> >> related to
>> >> http://lists.gnu.org/archive/html/emacs-devel/2010-10/msg00339.html
>> >
>> > That doesn't seem like a bug, but the fact that adding the box
>> > enlarges the size of the modeline, and likely the last line of the
>> > window isn't fully displayed.
>> 
>> Yes, that seems to be what's happening. But how is that not a 
>> bug? Even if the last line isn't fully displayed, can't emacs
>> try and figure out a way to respect the semantics of C-v and
>> M-v cancelling each other out in the middle of a file?
>> 
>> > Does it also happen if, instead of ":box t" you use ":box
>> > '(:line-width -1 :style released-button)"?
>> 
>> Yup, that fixes it, and even if I don't use :style released-button.
>>
>> > This gotcha should be pointed out more clearly in the doc, 
>> > since the default behavior increases the height of the
>> > current line, which can throw off various things.
>> >
>> > [And/or let's] change the default :line-width to -1 instead of 1. 
> 
> Right. That seems to be a different issue though. Even with a larger
> mode line, why do the scrolling commands behave this way?

My guess is that it is not a different issue.  As you guessed in citing the
October message (see e.g. the last message in that thread), as Juanma indicated
is likely, and as you seem to have confirmed using the fix he suggested, the
problem for the scrolling commands is also (I'm guessing) the increased
(mode-)line height.

Documenting this unfortunate side effect better, and changing the default so as
to not increase the line height, would (if our guesses are correct) directly
address your issue.