I have created GTK Grid using gtk_grid_new and set both column and row spacing to 0 (gtk_grid_set_column_spacing and gtk_grid_set_row_spacing), however for some reason I still get 1px spacing between rows.
I have even tried overriding css with * { margin: 0; padding: 0; } on whole window using gtk_css_provider_load_from_data, however it didn’t help.
On image you can see gap above purple rectangle (I have transparent background on window so it is easier to see)
