Safari has a "neat" feature that can auto-complete whatever you're typing into form input fields with information from your contacts, user names and passwords, credit cards, etc. This goes a little haywire in Gmail when you try to search for a label. In this case, the Gmail search popup shows up and over the top of that, the Safari popup shows up and blocks whatever Gmail was showing. The end result is rather irritating, not to mention that both popups show different information...
Fortunately there is a way to make this stop by disabling AutoFill for Contacts. This is done by disabling the Safari Preferences > AutoFill > Using info from my contacts option.
Once disabled, the label search feature in Gmail works as expected...
Now arguably this is Google's fault for not making Gmail handle AutoFill correctly. Although disabling this feature for a form field isn't as simple as setting an attribute on the input field, there are very well known and documented ways this can be done with cross-browser compatibility. For now though I'm happy to not have Safari use my contacts' information to auto-complete forms. 😎
-i