VHDL- increment and decrement push button - vhdl

Question

I am trying to increment and decrement with two push-buttons. The algorithm goes well but I have a little issue. Supposedly I am incrementing , when I try to decrement the accumulator it increments once more , and only after that it start
to decrement. Same if I try to decrement first. If it is someone to help me I will be very thankfully.
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.NUMERIC_STD.ALL;
use IEEE.std_logic_unsigned.all;
entity my_offset_controller is
port(clk : in std_logic;
input : in std_logic_vector(15 downto 0);
add_button : in std_logic;
sub_button : in std_logic;
output_res : out std_logic_vector(15 downto 0)
);
end my_offset_controller;
architecture Behavioral of my_offset_controller is
signal buttonState_up : std_logic:='0';
signal accumulator : std_logic_vector(15 downto 0);
signal lastButtonState_up : std_logic:='0';
signal buttonState_down : std_logic:='0';
signal lastButtonState_down : std_logic:='0';
signal buttonPushCounter : integer range 0 to 512 :=0;
process(clk)
begin
if rising_edge(clk) then
buttonState_up <= add_button;
buttonState_down <= sub_button;
if (buttonState_up /= lastButtonState_up) then
if (buttonState_up ='1') then
buttonPushCounter <= buttonPushCounter + 1;
accumulator <= std_logic_vector(to_unsigned(buttonPushCounter,accumulator'length));
end if;
lastButtonState_up <= buttonState_up;
elsif (buttonState_down /= lastButtonState_down) then
if (buttonState_down ='1') then
buttonPushCounter <= buttonPushCounter - 1;
accumulator <= std_logic_vector(to_unsigned(buttonPushCounter,accumulator'length));
end if;
lastButtonState_down <= buttonState_down;
end if;
end if;
end process;
output_res<= accumulator + input ;
This particularly module I use to controll the offset of an signal that I plot on an vga screen.

Answer 1

It's hard to help you without more information. You should provide a testbench with a chronogram to make it easier. Yet, by looking at your process I would say that the problem comes from the following lines:
buttonPushCounter <= buttonPushCounter + 1;
accumulator <= std_logic_vector(to_unsigned(buttonPushCounter,accumulator'length));
What you did there is increment buttonPushCounterat the same time that you update accumulator. This way buttonPushCounter will always be shifted by +1 or -1 depending on the last event.
What I can recommend is to update accumulator at every clock cycle instead of every time an event occurs. For example like this:
if rising_edge(clk) then
accumulator <= std_logic_vector(to_unsigned(buttonPushCounter,accumulator'length));
...