When will the tabId change

google-chrome, google-chrome-extension

Solution

By no means authoritative answer:

It has to do with pre-rendering, and that specific case may have to do with Instant search. As you type in the address bar, Chrome apparently pre-fetches and pre-renders some pages for faster navigation. Even if you do not trigger this via address bar, Chrome still can do tab swapping for performance reasons.

Consider the description of `chrome.tabs.onReplaced`:

Fired when a tab is replaced with another tab due to prerendering or instant.

Or, this remark in `chrome.webNavigation` API docs:

Not all navigating tabs correspond to actual tabs in Chrome's UI, e.g., a tab that is being pre-rendered. Such tabs are not accessible via the tabs API nor can you request information about them via `webNavigation.getFrame` or `webNavigation.getAllFrames`. Once such a tab is swapped in, an onTabReplaced event is fired and they become accessible via these APIs.

To summarize:

For performance reasons Chrome can spawn a separate, invisible tab, and swap an existing tab with this pre-rendered tab.

If that happens, a `chrome.tabs.onReplaced` event is dispatched, providing old and new `tabId`s.

Problem

Original source