I've just had an issue with Fossapup64 not allowing me to set the correct screen resolution for my Philips QHD monitor. It's native resolution is 2560x1440 but I was only getting 1920x1080. Never had this issue with Xenialpup or Bionicpup. Anyway, here's how to fix the problem if you have it.
Open a terminal and type the following, substitute in the resolution you are looking for if it's not 2560x1440:
cvt 2560 1440
You will get back the modeline information that will need for the next step, something like below:
- cvt_scrnsht.png (93.27 KiB) Viewed 1529 times
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Now, open the Screen/Graphics Wizard from the setup menu, click on the advanced tab and choose to edit xorg.conf manually.
Xorg.conf will open in a text editor. Edit the "Monitor" and "Screen" sections so they appear as below (obviously use the modeline info you got from cvt earlier if it's different from mine).
- xorgcfg_scrnsht.png (97.5 KiB) Viewed 1529 times
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Restart X to make the changes take effect. Don't forget to save your session before you reboot! 