The distinction between separable and inseparable verbs is sometimes less clear than one would think. Über- belongs to a group of prefixes (with durch-, um-, unter-, wider-, and to some degree hinter-) that can form both separable and inseparable verbs, sometimes from the same base verb.
It happens that such separable/inseparable pairs develop into completely different directions and become independent words with their own meaning (e.g. übersetzen when separable is ‘ferry over’, but when inseparable is ‘translate’). In some cases, however, usage is more fluctuant and both the separable and the inseparable form are preferred by some speakers. Regional, demographic or other factors may play a role here.
In such cases, even the same speaker may sometimes treat a verb as separable and sometimes as inseparable, either in the form of free variation (speaker alternates between er überreagiert and er reagiert über) or depending on how it is used (speaker always uses er überreagiert in the present tense, but überzureagieren in the extended infinitive). It appears that the überzureagieren form of the extended infinitive is far more common, but Google Books does find some evidence of zu überreagieren, too:
Zumindest aber können wir alle Zwänge relativieren und somit eine würdige innere Reaktion erhalten, anstatt zu überreagieren und zu viel Kraft zu investieren. (Andreas Gömmel, Das Glück zu leben!, n.p., 2002)
Eine andere Möglichkeit wäre, aggressiv zu werden – die andere Person anzugreifen, zu überreagieren oder gemeine Dinge zu sagen. (Karina Weichold/Rainer K. Silbereisen, Suchtprävention in der Schule, Göttingen, 2014)
Other examples of verbs that may be considered separable or inseparable include anerkennen (Warum wird „anerkennen“ teils als untrennbar betrachtet?) and überführen in the sense of ‘transport’ (Ihr Leichnam wurde in ihren Heimatort überführt/übergeführt), as well as the special case sandstrahlen (Warum schreibt man nicht »Sand strahlen«?).
