Grid Sidebar very large

I have set a column as hideable and hidden which results in a sidebar to appear.

Somehow is very wide when opened (the sidebar of the left grid is opened):

Any idea why?

I am using the valo theme and this is how I set up the hidden column:

Grid.Column column = grid.getColumn(propertyId);
if (column != null) {
  column.setHidable(true);
  column.setHidden(true);
}

29306.png