Download and install Mac OS X v10.5.7 or later to resolve this issue.
Mac Pro (Early 2009): Graphics distortion in Adobe Photoshop CS4 with certain window configurations
Mac Pro (Early 2009) computers may experience distorted graphics in the gray area of the image window if the window is set to a size larger than the document, or while toggling to full-screen mode. The distortion appears between the image and the border of the window containing the document—not within the image itself. The corruption may persist if the window is resized or moved; it may also happen when you open additional tabs in the Photoshop document window. This primarily occurs when opening large .PNG files.
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