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bug#34569: 26.1.90; Zero wide scroll bars
From: |
martin rudalics |
Subject: |
bug#34569: 26.1.90; Zero wide scroll bars |
Date: |
Tue, 19 Feb 2019 10:08:15 +0100 |
Setting 'scroll-bar-width' to zero can have unforeseen consequences
depending on the toolkit used. To reproduce with Emacs 26 run a Lucid
or Motif build as
emacs - Q --eval "(setq default-frame-alist '((minibuffer . nil)
(vertical-scroll-bars . nil) (scroll-bar-width . 0)))"
Then evaluate
(set-frame-parameter nil 'vertical-scroll-bars 'left)
Emacs aborts with a
X protocol error: BadValue (integer parameter out of range for operation) on
protocol request 12
GTK builds do not abort but have for example a scroll bar on the left
overwrite buffer text. A similar bug can be produced with horizontal
scroll bars.
It's not entirely trivial to explain why Emacs aborts here. The cause
is a combination of contrived logic and the absence of a more cautious
initial setting for X builds. For starters, frame.h says that
/* Width that a scroll bar in frame F should have, if there is one.
Measured in pixels.
If scroll bars are turned off, this is still nonzero. */
#define FRAME_CONFIG_SCROLL_BAR_WIDTH(f) ((f)->config_scroll_bar_width)
This comment is, unfortunately, wrong because in our example
'frame-notice-user-settings'
(setq parms (frame-parameters frame-initial-frame))
'frame-parameters' creates a zero entry for the 'scroll-bar-width'
frame parameter since our initial frame has no scroll bars and
'frame-notice-user-settings' passes that parameter on to 'make-frame'.
In frame.c this subsequently bypasses both checks in
x_set_scroll_bar_width (struct frame *f, Lisp_Object arg, Lisp_Object oldval)
{
int unit = FRAME_COLUMN_WIDTH (f);
if (NILP (arg))
{
x_set_scroll_bar_default_width (f);
...
}
else if (RANGED_INTEGERP (1, arg, INT_MAX)
&& XFASTINT (arg) != FRAME_CONFIG_SCROLL_BAR_WIDTH (f))
{
FRAME_CONFIG_SCROLL_BAR_WIDTH (f) = XFASTINT (arg);
...
since ARG is zero which is neither nil nor a ranged integer > 0. So
we continue with the initially zero FRAME_CONFIG_SCROLL_BAR_WIDTH (f)
and try to make a window with a zero wide scroll bar produced from the
initial w->scroll_bar_width (which equals -1) and these definitions in
window.h:
/* Width that a scroll bar in window W should have, if there is one.
Measured in pixels. If scroll bars are turned off, this is still
nonzero. */
#define WINDOW_CONFIG_SCROLL_BAR_WIDTH(W) \
(W->scroll_bar_width >= 0 \
? W->scroll_bar_width \
: FRAME_CONFIG_SCROLL_BAR_WIDTH (WINDOW_XFRAME (W)))
/* Width of scroll bar area in window W, measured in pixels. */
#define WINDOW_SCROLL_BAR_AREA_WIDTH(W) \
(WINDOW_HAS_VERTICAL_SCROLL_BAR (W) \
? WINDOW_CONFIG_SCROLL_BAR_WIDTH (W) \
: 0)
Here the first comment is wrong again, WINDOW_CONFIG_SCROLL_BAR_WIDTH
is zero and X is rightfully annoyed.
The easisest fix I could come up with is switching the two branches of
the if clause in x_set_scroll_bar_width thusly:
void
x_set_scroll_bar_width (struct frame *f, Lisp_Object arg, Lisp_Object oldval)
{
int unit = FRAME_COLUMN_WIDTH (f);
if (RANGED_INTEGERP (1, arg, INT_MAX)
&& XFASTINT (arg) != FRAME_CONFIG_SCROLL_BAR_WIDTH (f))
{
FRAME_CONFIG_SCROLL_BAR_WIDTH (f) = XFASTINT (arg);
FRAME_CONFIG_SCROLL_BAR_COLS (f) = (XFASTINT (arg) + unit - 1) / unit;
if (FRAME_X_WINDOW (f))
adjust_frame_size (f, -1, -1, 3, 0, Qscroll_bar_width);
SET_FRAME_GARBAGED (f);
}
else
{
x_set_scroll_bar_default_width (f);
if (FRAME_X_WINDOW (f))
adjust_frame_size (f, -1, -1, 3, 0, Qscroll_bar_width);
SET_FRAME_GARBAGED (f);
}
XWINDOW (FRAME_SELECTED_WINDOW (f))->cursor.hpos = 0;
XWINDOW (FRAME_SELECTED_WINDOW (f))->cursor.x = 0;
}
But maybe someone has a better idea.
Note that Windows builds sidestep the problem by unconditionally doing
/* By default, make scrollbars the system standard width and height. */
FRAME_CONFIG_SCROLL_BAR_WIDTH (f) = GetSystemMetrics (SM_CXVSCROLL);
in 'x-create-frame'.
Note also that with emacs 27.1 the bug can be produced more directly by
including
(setq default-frame-alist '((vertical-scroll-bars . nil) (scroll-bar-width .
0)))
in the early-init.el and then enabling vertical scroll bars. The
indirection via 'frame-notice-user-settings' is not needed there.
Thanks, martin
- bug#34569: 26.1.90; Zero wide scroll bars,
martin rudalics <=
- bug#34569: 26.1.90; Zero wide scroll bars, Eli Zaretskii, 2019/02/23
- bug#34569: 26.1.90; Zero wide scroll bars, martin rudalics, 2019/02/23
- bug#34569: 26.1.90; Zero wide scroll bars, Eli Zaretskii, 2019/02/23
- bug#34569: 26.1.90; Zero wide scroll bars, martin rudalics, 2019/02/24
- bug#34569: 26.1.90; Zero wide scroll bars, Eli Zaretskii, 2019/02/24
- bug#34569: 26.1.90; Zero wide scroll bars, martin rudalics, 2019/02/24
- bug#34569: 26.1.90; Zero wide scroll bars, Eli Zaretskii, 2019/02/24
- bug#34569: 26.1.90; Zero wide scroll bars, martin rudalics, 2019/02/25
- bug#34569: 26.1.90; Zero wide scroll bars, Eli Zaretskii, 2019/02/26