Users should consider changing a small number of default preferences provided by TeXShop. Some of these changes depend on characteristics of individual displays and cannot be set until you use the program.

By default, TeXShop opens source and preview windows in the spot they last appeared. This only makes sense when you deal with one document at a time. It is usually better to open source and preview windows in a predetermined optimal position. To make that happen, typeset a simple document. Then arrange the position and size of the source and preview windows. Side by side is best if your screen is large enough. Open TeXShop preferences. In the "Document" tab, select "All Windows Start At Fixed Position" and click "Set With Current Position." Make the same change in the "Preview" tab.

How do I configure TeXShop?
Recommended Preference Changes