Woah, guys, i made it. Such a trivial edit, lol.
upperPanel.setViewport({0, 0, 0.75f, 0.025f}); //Set panels ratios.
mainPanel.setViewport ({0, 0.025f, 0.75f, 0.975f});
rightPanel.setViewport({0.75f, 0, 0.25f, 1});
Method for setting size of panel (view). Fix is to std::floor calculation result
void OperatingSystem::adjustPanel(const sf::RenderWindow& win, sf::View& view)
{
view.reset({0.f, 0.f,
std::floor(win.getSize().x * view.getViewport().width), //std::floor it!
std::floor(win.getSize().y * view.getViewport().height)}); //this too
}