Question about screen resolution, dpi and font size

[d_stranz]

Your image links don't work in Firefox. Why didn't you simply upload the images to the forum? There's even an "Insert Image" icon to do that...

However, I think I know what you mean. I use a 4K monitor (3840 x 2160). Normal Windows fonts are too small for me to read easily, so I used Windows "Customize your display" settings to increase the font size to 125% of normal. I noticed that some of my Qt apps did not react to this change properly, possibly those running with Qt 5.4, and especially those where I had designed GUI elements with fixed maximum sizes.

Ask your user if that is what he has done. You can simulate it (in Windows, anyway) using Settings->Display and increasing the text size. My fix was to either remove the size restrictions on labels, pushbuttons, etc. or to increase them so the text would fit when viewed on my screen. With any new GUIs, I use layouts wherever possible, and give them room to grow if necessary by adding horizontal or vertical stretch elements. In Qt 5.5 at least, I haven't noticed any further problems.