Hi,
DF v9.7
Windows 10,
Firefox 78.0.1 (64-bit)

in Jan 22, 2019 you wrote the following in #19

"We're planning to add an option to choose the middle-click behaviour at some point, but for now, you could add the "Move Window to Next Monitor and Size Proportionally" function as a TitleBar Button, and that should size the window like you had before."

Is there any additional setting needed for it to size proportionally?

Then you have to have the ”Title Bar” on!
When you use at 13-14" laptop you need space. 'Title Bar' in FF is unnecessary.

The middle-click move seems to work ”Move Window to Next Monitor” but it can not Size Proportionally.