What exactly triggers a need to reset in visual editor

Reset is required when PHPRunner is not able to merge template changes made in Visual Editor with project changes.
For example, you made some changes to your project i.e. added a few new tables and turned on security. This would add a menu and a 'sign out' link to all list pages. In normal scenario PHPRunner analyzes modified page and finds where new items need to be inserted. If page were heavily modified in Visual Editor merge becomes impossible and the only way to resolve the conflict is to reset page in Visual Editor.
The most common scenario is opening a highly customized project created by one of previous versions of PHPRunner. Breaking project created by current version of PHPRunner is much more difficult though still possible.
If you are interested in how merge works start with this Wikipedia article.

Thankyou Sergey, very interesting. That will help.

I know it must be a nightmare bit of code back there, but it would be nice to be able to recover from a reset somehow. It can be a lot of work lost.

I have had to redo some panels a lot, but as you say, I am building the database needs as I go, I always end up wanting "Just one more field" or a new table for a new feature. Anyway, thanks.

Cheers.

Agreed Dale... I have had a lot of formatting work lost when PHP Runner required a "reset" ... I therefore don't spend any time customizing in the visual editor, since I know all that time spent is likely going to be lost again...
Sergey... this would be high on my wish list for the next version of PHP Runner if it's at all possible to be able to retain customizations upon reset.

To be able to retain formatting have been high om my priority list since version 1.2 or somewhere around that. And you will find numereus posts here saying the same.
I appreciate that it may not be an easy task, but it would be fantastic