Premature layout cycle in silverlight

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Silverlight4

Silverlight5

I have a Silverlight application which seems to be executing the measure method of a custom panel prematurely.

My understanding of the cycle in silverlight was this:

• Wait for user action
• User clicks button etc
• --->Event --> Your code
• Measure of any controls which were invalidated
• Arrange of any controls which were invalidated
• Wait for user action again

Is this correct? This seemed to be correct until I upgraded from SL5 beta to SL5 (non-beta).

What is now happening is that, halfway through one of my methods, measure of a control occurs instantly when a different & unrelated control has its IsEnabled = false.

The measure doesn't occur when I put a break point on the IsEnabled = false line (?!?), and all code is on the UIThread / synchronous.

Is this behaviour by design?

I can't seem to replicate this in a demo project but I'm happy to supply more info if required.