Applets on MacOS over 800 pixels high have display problems
Oracle bug report # 9002483
A secondary issue is that the "Change the text on the buttons" button doesn't respond, but that is OK if the applet area is smaller, as illustrated at this link.
The display problem occurs for both Firefox and Safari on MacOS. Everything is fine on Windows. Source code for the applet is at this link.
Addendum: the problem, reported in JRE 1.7.0_21 is gone in JRE 1.8 b88. It is also gone by JRE 1.7.0_45.
If you have any comments please contact Mickey Segal. A listing of many Java resources is at this link.