- Edited
[Editor] Maximize doesn't work properly on vertical res
On my dual monitor setup, I usually have my left monitor be landscape, and my right screen be portrait. Works wonders for coding.
Just noticed that double clicking the title bar, and clicking "Maximize" incorrectly size the window when I have Spine in the portrait monitor. Clicking on "Fullscreen" does work properly, however.
Lastly, on Windows 7, I cannot drag the window to an edge of the screen to resize it. I realize this might be due to some of the cross-platform stuff you're doing.
Since there's workarounds for all of this, no big deal, but I figured I'd mention them. 
Spine actually runs on a Java VM so it's cross-platform. It has its own set of peculiarities, and even more depending on the framework being used for managing a windowing system for the program.
As it is, I can't even associate the .spine file extension to open in Spine when I double-click it in Explorer. I dunno if it's a bug or an unimplemented thing or just a limitation of Java. Or just hard to do.
Aero Snap is handy though.
I can't remember how Eclipse behaves when it comes to these Windows features.
I haven't had multi mon for a long time so haven't been able to test Spine with it. 
3 years later
Ran into this, still happens on 3.4.02 Professional (launcher 3.4.04.)
Hitting "Maximize" positions the top left corner of the window onto whichever screen it takes up the most area on, which is sane (and part of the default windows behavior.) However, the window is size is changed to whatever the size of the primary monitor is set to, instead of whichever monitor it was maximized to.
If I had to guess, I'd assume you're manually positioning & sizing the window in code to e.g. fix it's position after switching to a borderless / unresizable window mode while maximized. If you can change your code to set borderless/unresizable window mode immediately before maximizing instead of afterwards, manually positioning & sizing the window should be unnecessary? Although, Win+{Left,Right} are also broken, so perhaps I'm underestimating how much custom, manual code you have for your window's skin.
Wow, a thread from 2013.
Anyway, I agree. Snap features are useful. Side-by-side is useful for anyone. Windows 10's snap assist is especially useful.
They're not supported yet though, still. I wonder how easy it is to implement, actually.