prevent Notepad++ from resizing when a new document is opened
This a work-around but it is a successful work-around: when the window gets 'unsnapped' on document open it resizes it to the size the window was before you snapped it. Ergo if you drag the window to the size of half your monitor, then snap it to the side (or just leave it, no need to snap it at this point) when it 'unsnaps' it basically stays in the same position.
Again, a work-around but one that works for me.
Here's an easy workaround:
Just drag your files into the N++ window to open them (instead of double-clicking, right-click & open with, etc.).