How can I stop that the viewport is activated when I want to drag and drop a material into the material editor?
The situation is now that when I select a material, it activates the viewport, en overrides the material editor, and brings the viewport on top, with other words can not drop the material in the editor.
It sounds like you are trying to drag a Texture from the Content Browser into the Material Editor, is that correct?
While it is true that clicking on any free-floating window will bring it to focus, this should not cause any issue depending on your UI setup or workflow.
For example, if you keep the Content Browser docked, and the Material Editor as free-floating (as seen in the screenshot), you can drag from the CB into the ME without any issue.
Another very quick option (if dragging is not a convenient workflow for you) is that you can select your texture in the CB, and then back in the ME simply hold down T on your keyboard and mouse-click on the graph. A texture sample node will be placed with your selected texture already included.