This series of changes improves the toolbox layout:

• Remove the behaviour of placing a section on the last row of the previous section.
• Change the minimum size calculation so that it works when the toolbox is laid out horizontally.
• Wrap the toolbox in a scroll widget to allow scrolling the toolbox when there isn't enough space.
• When the toolbox docker is floating, change the toolbox orientation according to the size of the toolbox.

The individual commits can be viewed here: https://github.com/KDE/krita/compare/master...alvinhochun:alvin/toolbox-layout
You can fetch my branch by running git fetch https://github.com/alvinhochun/krita.git alvin/toolbox-layout