Running:
• Lion 10.7.5
• Spectacle 0.8.6
When hitting my maximize-window key combination, Spectacle now makes my Terminal window larger, centers it, but still leaves a space around the edges. This doesn't happen in any other apps that I've tested (e.g. Mail, Chrome, etc). This hasn't happened before but I have no way of confirming as I don't know how to roll back to previous releases. I'd be happy to test previous releases if I knew where to download the binary. I went to github (i.e. here) but realized I didn't have the tools for compilation.
Here's a screenshot:

This may be related to #243.
I figured out how to download 0.8.5 and I can confirm that this bug doesn't occur in that version.
I have the same problem with iTerm2
This is expected behavior in 0.8.6. See pull request #250.
If it's expected behavior, it's inconsistent. First, telling spectacle 0.8.6 to create a full-sized half-pane window on the right or left fills the screen as expected; it doesn't do the bordered centering that #250 does when you want to do a full-size window resize. Further, why does this only happen with Terminal/iTerm? I would expect this behavior in all other apps too. The behavior I'm describing happens in Lion (10.7).
I tested this in Yosemite and don't see the described behavior (though I do see another bug). My suggestion is that you push 0.8.6 to 10.9 and higher and mark 0.8.5 for 10.7 and before. (I'm not sure about 10.8 as I don't have a 10.8 system around) and make the changes in the README. Just in case anyone else is looking for a solution, I suggest staying at 0.8.5.
If Spectacle is able to precisely resize a window it will do so. Some Apps, like Terminal and iTerm, restrict the sizes of their windows so that no rows or columns in the terminal are truncated. You can see this when you manually make these windows larger or smaller.
In previous versions of Spectacle these windows would just overlap, or awkwardly fit in their new positions. In 0.8.6 Spectacle will resize any window to fit its new bounds, but only if necessary.
Honestly, in Lion, 0.8.5 behaves just fine. I recognize this may not be true of other MacOS X versions though...
Avery
On Oct 27, 2014, at 9:45 PM, Eric Czarny wrote:
If Spectacle is able to precisely resize a window it will do so. Some Apps, like Terminal and iTerm, restrict the sizes of their windows so that no rows or columns in the terminal are truncated. You can see this when you manually make these windows larger or smaller.
In previous versions of Spectacle these windows would just overlap, or awkwardly fit in their new positions. In 0.8.6 Spectacle will resize any window to fit its new bounds, but only if necessary.
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Seems to have returned in El Capitan -- anyone else having this issue?

It's not returned, it never went away. It's expected as per @eczarny a few comments up.
@kokarn I've read the linked issue, but I still don't understand why iterm2 can't fill the screen. This is honestly the only thing that I dislike about iterm2.
For iTerm2
You can turn off snap to grid permanently by turning on Prefs>Advanced>Terminal windows resize smoothly.
https://gitlab.com/gnachman/iterm2/issues/4265
EDIT
The clearer way:
$ defaults write com.googlecode.iterm2 DisableWindowSizeSnap -integer 1
@twlz0ne YES! I LOVE YOU! ❤️
Most helpful comment
For iTerm2
https://gitlab.com/gnachman/iterm2/issues/4265
EDIT
The clearer way: