If all margins and spaces are set 0 to QGridLayout, there are still like border 1px in top/bottom and right/left sides. Then widgets on grid layout are never stick each other even calling below methods to set 0.

    ui->centralwidget->setContentsMargins(0, 0, 0, 0);
    ui->gridLayout->setSpacing(0);

Expectation is those widgets on grid layout are sticked, but it seems there is no way for that. 

Please see attached image and sample project.