Question: They're more noticeable when in maximize windows:

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> [![enter image description here](https://i.stack.imgur.com/YLX5v.png)](https://i.stack.imgur.com/YLX5v.png).
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Answer: 
These borders are part of the default Windows 7 windows appearance. Eithe set your border to 0 pixels width or use an alternative style.

Both may require hacks, and will not be limited to just IrfanView
As of Version 4.60 (Release date: 2022-03-18) there is a shortcut that will permit "thin" borders, in a way similar to the original request:

New hotkey: `ALT` + `SHIFT` + `B`: Show thin or normal border (current session only)