Sorry my bad, the diff is reversed. Use the following commands instead:
mkdir tmp cd tmp sudo apt install devscripts build-essential fakeroot dpkg-dev sudo apt-get build-dep xfwm4 apt-get source xfwm4 cd xfwm4-4.12.5 patch -R -p1 -i xfwm4_slack_fix.diff # !!! Point it where the patch is located!!! debuild -b -uc -us sudo dpkg -i ../xfwm4_4.12.5-1ubuntu0.18.04.1_amd64.deb
Sorry my bad, the diff is reversed. Use the following commands instead:
mkdir tmp fix.diff # !!! Point it where the patch is located!!! 4.12.5- 1ubuntu0. 18.04.1_ amd64.deb
cd tmp
sudo apt install devscripts build-essential fakeroot dpkg-dev
sudo apt-get build-dep xfwm4
apt-get source xfwm4
cd xfwm4-4.12.5
patch -R -p1 -i xfwm4_slack_
debuild -b -uc -us
sudo dpkg -i ../xfwm4_