Interestingly Chrome use to support always on top windows in extensions. They eventually got removed

There is a very interesting comment from one of the developers on the removal bug (https://bugs.chromium.org/p/chromium/issues/detail?id=467808):

> While closing this issue as "Fixed", I feel I need to offer a couple of words of personal explanation to the users of Trello who were directed here by the developer of the app, as well as others who can be interested.

> I'm the original developer of Panels in Chrome, and I'm the engineer who's lead a team to maintain them. I also finally removed them. Panels were a part of my life in form of time, thoughts, inspiration and passion. Our initial plans were to enable panels to all apps which wanted to use them, and we saw how those can be useful of course. It is with sadness but also with appreciation of the experience that I had to remove the feature from Chrome M54.

> The time passes and things change, we all learn. In case of Panels, we "proved" by practice that it takes a team of a few engineers full time to be able to catch up with teams of OS developers in Windows, OSX, Linux and even our own ChromeOS. The window management and graphics/input subsystems are constantly evolving and it is more or less prohibitively expensive for a small team to try to build and keep a high quality but non-standard window management mode. OSes have too many mechanisms that are linked to a specific windows behaviors (focus, window switching, active windows treatment, titlebars, where input goes, shortcuts, animations, multiple desktops, other OS gadgets etc), and usually OSes do not provide 'hooks' or APIs to integrate with those, which makes it necessary to 'reverse-engineer' and hack around. While it can be done, it quickly leads to 'card house' design that falls down even easier with the next major OS update... So the costs of development and testing rise.

> There was another factor that kept us from enabling panels for all apps. The specific UI Panels provide does not scale well if/when multiple apps are using it. It is very easy to get in trouble with multiple always-on-top windows popping up from multiple apps. Several independent apps using panels at the same time actually makes the interface annoying and hard to control. We had several apps using Panels at the same time and it was hard to manage windows because they were all intermixed. There were several design ideas on how to build some 'panel managers' and corresponding APIs but they increased the complexity significantly, especially considering those would be in addition to APIs OS provides for other windows. So we never were able to enable it for every app.

> Now, if the major OSes would adopt the concept and built into their window managers, it'd be much much easier to support it in Chrome. But it's not the case, except for ChromeOS where panels stay.

> So at the end, this was a great experiment. We are now way past the point where additional discussion can move the needle much. Google Hangouts app had millions of users and some of them loved Panels. Apparently Plus for Trello has passionate followers. The YouTube viewers is a neat idea. But we all need to move forward, the platforms change much faster and generally to the better :) Hope developers of your favorite apps will figure out even better ways to implement great UI their users demand!