On very large displays and above all in multi-monitor configurations, could be very useful to have not only Developer Tools into a separate window (to place it syde-by-side to browser) but maybe also to display more than one of its panels at the time.
For example, imagine to have the website on the primary monitor and then multiple panes of Developer Tools onto secondary one.
I tried to imagine a possible (mockup) solution in this image:
It could be very useful to have many panels (also more than 4) contemporary to avoid need of continuous switch. For example, copy&paste new CSS file and in the same time can view updated style into inspector