In testing, I've found if you are using an on timer script and a web viewer, if you scroll the webviewer on a Windows computer the on timer will stay suspended after you stop scrolling. The on timer will on resume if you click on the webviewer or move the mouse outside the webviewer. On a Mac the on timer will resume as soon as the scrolling stop. I'm assuming this is a bug and will report this to FileMaker. I've attached a file demonstrating this. If anybody has encountered this bug, have you been able to work around this unexpected behavior?
I solved this issue by using FileMaker 11 instead of FileMaker 12. FileMaker 11 behaves exactly as expected when trapping for the Alt key. There definitely is a bug in FileMaker Pro 12 (Win).