In recent state creating box widget requires four steps: 


Reproducible: Always

Steps to Reproduce:
1. move the mouse to the toolbar and select the box tool
2. left click on the diagram to position the top left corner of a fixed sized box on the diagram
3. select the box
4. move the mouse into the bottom right corner to resize the box to the required size



Expected Results:  
Better usability will be provided with 
1. select the box tool with a shortcut 
2. left click on the diagram to position the top left corner and on left mouse button hold and mouse move position the bottom lower corner of the box until left mouse button has been released