Notepad++ : how to reattach the "find result" box?

Method1:

Double click the title bar of the separated result box. It should return to its default docked position. (you have to double click where the red cross on the screenshot is)

Method2:

Drag it by by the title bar down towards the lower border of the window as shown on this screenshot:

Until the border showing the default position of the results box appears as shown on this screenshot:

You have to drag so that your mouse cursor is precizely on the edge of the status bar of NP++ main window

Release the mouse button... Done!

Just drag the search results window to the bottom (onto the status bar) of the Notepad++ main window and there should appear a gray line. If you release the mouse button, the search results windows should dock again.