Behavior CONFIG.SHAREDSPACES differs between v3.6.5 and v4.3

I'm using following line in my custom config file:

config.sharedSpaces = { top: 'editorTop' };

Using this, in v3.6.5., I could resize the content section of the editor without resizing the toolbar section.
But in v4.3, the content section is fixed to the toolbar. Resizing the editor content section also resizes the toolbar section and the toolbar items start to wrap around.

How can this be solved?

Thx