set WM_NORMAL_HINTS program specified resize increment for any client (e.g. alacritty)

resize-increment.c

/*
 * Author: intrntbrn
 *
 * Compile with:
 * gcc resize-increment.c -Wall -o resize-increment `pkg-config --cflags --libs x11`
 *
 * Usage:
 * resize-increment windowid width_inc height_inc
 * e.g.: resize-increment 1234567 6 12
 */

#include <X11/Xatom.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

static void SetWmSizeHints(Display *display, Window window, int width_inc,
                       int height_inc) {
  XSizeHints *hints = XAllocSizeHints();
  hints->flags = PResizeInc;
  hints->width_inc = width_inc;
  hints->height_inc = height_inc;

  XSetWMSizeHints(display, window, hints, XA_WM_NORMAL_HINTS);
  XFree(hints);
}

int main(int argc, char *argv[]) {
  Display *display;
  Window window;
  int width_inc, height_inc;

  display = XOpenDisplay(NULL);
  if (display == NULL)
    return 1;

  window = 0;
  if (argc > 3) {
    sscanf(argv[1], "0x%lx", &window);
    if (window == 0)
      sscanf(argv[1], "%lu", &window);

    sscanf(argv[2], "%u", &width_inc);
    sscanf(argv[3], "%u", &height_inc);
  }

  if (window == 0)
    return 1;

  SetWmSizeHints(display, window, width_inc, height_inc);

  XCloseDisplay(display);

  return 0;
}

Shellscript to autospawn resize-increment for a specific process ( alacritty in this example):

#!/bin/bash
alacritty & pid=$!
i=0
while
	windowid=$(xdotool search --pid $pid)
	:
	i="$((i+1))"
	[ -z "$windowid" ] && [ "$i" -lt 100 ]
do :;  done
[ -z "$windowid" ] && exit 1
resize-increment "$windowid" 6 12

You may need to adjust the limit of$i (100).