A lot of us probably noticed already that FGO isn’t optimized for newer phones, in terms of resolution.
Older phones still have the screen ratio where if you take screenshots or screen record, it will end up as 1280 x 720 , 1920 x 1080 , etc.
Taking a screenshot on a newer phone will make the picture 2340 x 1080 , 2220 x 1080 , etc. depending on the phone.
Phones usually have an option to edit pictures ; look for the pencil icon.
If you pick “crop” there should be an option for 16:9 orientation.
Selecting it will give a rectange of fixed size that you can manually move around on screen.
Double tapping the rectangle might auto-center the rectangle area, but not always. I tried two different phones and it only works for one of them. So if unlucky, you will have to adjust it manually.
disclaimer: This is really only going to matter to people with OCD.
If seeing the blue borders doesn’t really bother you, well yeah.
And someone might have already found a less tedious solution.