(12-22-2017, 06:26 AM)Blighty Wrote: Windows > Dockable Dialogues > Tool Options
It probably opens as a separate window. To dock it: Don't drag the title bar of the window. Do drag on the tab title.
Same as @blighty. It is the same as in Chrome & Firefox where to move a tab to another window, you drag the tab in the window and not the title bar:
(click for animation)
Re-installing Gimp doesn't help, because all your set up in your Gimp Profile, which is not overwritten when you re-install. So instead of re-installing Gimp, just erase/rename the profile to have Gimp recreate on with default values. But keep this solution for the real bad cases, because to recover the initial windows you can also go to Edit>Preferences>Window management>Reset saved window positions to default values.