Is there a resizeable layout

My application shall basicly have a similar layout as pgAdmin (see attachment): Left side a tree widget with one column, and right side a stacked widget with changing content depending on what is selected in the tree view.

What kind of layout is available in the toolbox to have a vertical bar between the tree view and the stacked widget to resize the space occupied by the two with the mouse at runtime?

You need a splitter (third or fourth item in the "Form" menu).

Oh, I overlooked it completely - maybe because those splitter layouts are allways disabled (greyed out). Are they not applicable on the top level (main window), or are there any other constraints?

Edit:

Stupid me, the two related widgets must be selected to enable the sptitter layouts of cause.

Thanks!