The reason the system doesn't save automatically is to account for "poorer" network conditions. A lot of our users are on the go on iPads - so they were running into problems where the constant saving with each change wasn't always able to keep up with their network. Having the "Save" overlay is to try and make sure the Estimate gets synced to our servers. We're trying to make data integrity a bit better. Though it's worth mentioning that if you "forget" to save, we do have a "rescue" version of it. So if you were to try making an Estimate, and then close your browser, it should open back up to that estimate as it was!
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