The easiest way to fix it is to modify either the source of rxvt or the source of openbox and to recompile -- look at the bottom of this answer for two very simple patches.

Another way is use an LD_PRELOAD hack that will turn off the PResizeInc flag when getting the WM_NORMAL_HINTS window property, and will prevent it from being turned on when setting that property. This will only affect the width_inc and height_inc properties -- programs that set their minimum window size or its aspect ratio will continue to work fine.

$ cat no_inc_size_hints.c #define _GNU_SOURCE #include <dlfcn.h> #include <X11/Xlib.h> #include <X11/Xutil.h> Status XGetWMNormalHints(Display *dpy, Window w, XSizeHints *hp, long *rp){ static typeof (XGetWMNormalHints) *orig; Status r; if(!orig) orig = dlsym(RTLD_NEXT, "XGetWMNormalHints"); if((r = orig(dpy, w, hp, rp))) hp->flags &= ~PResizeInc; return r; } void XSetWMNormalHints(Display *dpy, Window w, XSizeHints *hp){ static typeof (XSetWMNormalHints) *orig; if(!orig) orig = dlsym(RTLD_NEXT, "XSetWMNormalHints"); hp->flags &= ~PResizeInc; orig(dpy, w, hp); } $ cc -shared -fPIC -Wall no_inc_size_hints.c -ldl -o no_inc_size_hints.so 

Then urxvt or openbox will have to run with the

LD_PRELOAD="$LD_PRELOAD /absolute/path/to/no_inc_size_hints.so" 

variable in their environment. Example:

$ LD_PRELOAD=`pwd`/no_inc_size_hints.so openbox --replace 

There are two problems with this:

1. urxvt is usually installed as a utmp setgid binary, and the LD_PRELOAD variable is cleared when exec'ing a set[gu]id binary. Look in the man page for why the utmp permission is (not) needed. So you'll have to copy the urxvt executable elsewhere (which will turn the setgid bit off). Example:

   $ cp which urxvt .; LD_PRELOAD=pwd/no_inc_size_hints.so ./urxvt

2. The window manager is usually called through ssh-agent from the /etc/X11/Xsession.d scripts on many systems, and ssh-agent will also wipe the LD_PRELOAD variable from the environment. So something like this may be needed:

   cat <<'EOT' >/etc/X11/Xsession.d/98-no_inc_size_hints

   export LD_PRELOAD="$LD_PRELOAD /path/to/no_inc_size_hints.so" case $STARTUP in /usr/bin/ssh-agent*) STARTUP="/usr/bin/ssh-agent env LD_PRELOAD=$LD_PRELOAD ${STARTUP#* }";; esac EOT

Possible patch for openbox-3.6.1:

 --- openbox/client.c~ 2018-10-06 08:34:25.615967414 +0300 +++ openbox/client.c 2018-10-06 08:34:28.916133702 +0300 @@ -1757,9 +1757,6 @@ void client_update_normal_hints(ObClient if (size.flags & PBaseSize) SIZE_SET(self->base_size, size.base_width, size.base_height); - if (size.flags & PResizeInc && size.width_inc && size.height_inc) - SIZE_SET(self->size_inc, size.width_inc, size.height_inc); - ob_debug("Normal hints: min size (%d %d) max size (%d %d)", self->min_size.width, self->min_size.height, self->max_size.width, self->max_size.height); 

and for rxvt-unicode-9.22:

 --- src/main.C~ 2018-10-06 08:33:08.580085731 +0300 +++ src/main.C 2018-10-06 08:33:37.549545455 +0300 @@ -657,7 +657,7 @@ rxvt_term::window_calc (unsigned int new unsigned int w, h; unsigned int max_width, max_height; - szHint.flags = PMinSize | PResizeInc | PBaseSize | PWinGravity; + szHint.flags = PMinSize | PBaseSize | PWinGravity; szHint.win_gravity = NorthWestGravity; /* szHint.min_aspect.x = szHint.min_aspect.y = 1; */ @@ -1073,7 +1073,7 @@ rxvt_term::resize_all_windows (unsigned { szHint.flags &= ~(PBaseSize | PResizeInc); XSetWMNormalHints (dpy, parent, &szHint); - szHint.flags |= PBaseSize | PResizeInc; + szHint.flags |= PBaseSize; } if (!ignoreparent) 

Both are against the versions from debian 9.5; they'll only apply with patch -l -- I don't know how to get this damn thing to preserve tabs.