I may have stumbled across something similar to this issue. I wrote my report writer session for DevCon over a month ago and all my samples ran just fine. Last night I presented it to a user group and some of the samples went into an infinite loop of flickering the preview over and over again. These were samples using an extension handler with some new buttons added to the toolbar. I tried to think of what I have changed on my system and I remembered that 2 weeks ago I changed my resolution and set up a dual monitor system on my laptop with a new driver for the 2nd monitor. I'm wondering if that somehow is causing this problem and interfering with Fox.

I wish I had an answer for you, Cathy.

You could try and recompile the ReportPreview.App with DEBUG_METHOD_LOGGING=.T. and watch the Debug Output window, perhaps notice a sequence of events that is unexpected. Of course doing this changes the window stack and may prevent or exacerbate the error conditions.

Sounds like you're not doing anything with ExtentionHandler.paint() or .RenderPages() which is usually the culprit for this kind of thing...