It currently has 30k active users, and 5 star score with 119 reviews. I cannot confirm whether it's a clean extension or not. So it does NOT indicate that the extension IS spying.
However, reading history seems like something that every tab-related extension asks for, and is probably just a side-effect of requesting the API for 'managing windows'. But it can read your browsing history (all visited URLs). At least it doesn't ask for 'Read contents of the active webpage' so at least it cannot spy on secure text you've typed on pages. When installing the extension, it asks for 1 permission: 'It can: Read your browsing history'. But I do not endorse any extension since I have not tried it and don't know if it contains spyware (which is a risk that you always take with browser extensions). I have not tried it, but here is one extension for merging all windows into 1 window: