@Joanna-Szpakowska
i can, unfortunately, not reproduce what you are experiencing.
i have tested your resolution 2560x1440, as seen at the screenshot below, as well as all higher resolutions up to 5k (5120 × 2880).
all resolutions work fine and as expected.
Imgur
for further testing, could you please:
download this specific notepad++ 7.6.4. portable version from
>>> here <<<.
(this portable version will not interfere with your installed notepad++ version and does not require any installation)
extract npp.7.6.4.bin.x64.zip to your desktop.
open the extracted npp.7.6.4.bin.x64 folder.
start this portable version by double clicking on notepad++.exe inside this folder.
(note: notepad++.exe will just be called notepad++, if you have enabled to hide all known file extensions at your explorer settings)
does the portable version give you the same results as your installed version, or does it differ ?
if this portable version behaves the same on your machine, you could try to set the windows scaling to 100%, as seen at the screenshot above, to try if it makes any difference, and/or change your resolution temporarily to 1920x1080 for testing.
if it does not make any difference, the next thing would be to look whether an updated driver is available for your graphics card.
note: your experience might not be directly related to notepad++ itself, but to some lesser commonly known issue on your specific machine.