Currently I have my program organized this way in QTDesigner (which I use with VS 2022): QMainWindow->centralWidget(QWidget)->QTabWidget->Tab(QWidget)->QGridLayout. All these elements are created in QtDesigner. In my cpp code I'm downloading some data and generating QTableWidget* m_table. Unfortunately after adding it to QGridLayout element, I get a black border exactly around this layout. How it's possible if it border can't be set at all for this element, at least in QtDesigner?
QTableWidget* m_table;
///(...)
ui.gridLayout->addWidget(m_table);
Look into Qt's stylesheets. You should be able to style your QTableWidget in any way you want to.
Keep in mind a QTableWidget is specialized version of QTableView, so you have set your stylesheet to a QTableView:
m_table->setStyleSheet("QTableView {background: transparent; border: 1px solid green;}");
Here are some links to get you started with Qt stylesheets:
https://doc.qt.io/qt-5/stylesheet.html