See sliver at the top. Similar issue happens with Chrome, iTerm. http://i.imgur.com/9CmqZMB.png
I'm experiencing this too, except it's at the top of Terminal.app windows specifically.
Fullscreen otherwise works properly for all other applications I'm using.

@zeke I've had the same issue with iTerm. It's extra noticeable since Spectacle doesn't seem to think there is enough space to expand Terminal by one more row and Terminal is fixed in multiples of a character's width/height.
Update with my stats.

@SinOverCos, same here.
This also happens sometimes at the bottom of the screen instead. I've experienced this with Preview.
Also experiencing this issue.
Having the same issue
Same
I had this issue with Spectacle 1.2 OS X 10.12.6 using Terminal.app. I thought it was a bug in Spectacle for the longest time, but I just checked and Terminal indeed cannot accommodate and more rows, so Spectacle makes it as large as possible and then centers it vertically.
It might look a bit better if Spectacle made Terminal flush with the top of the screen instead of centered vertically in this case. Beyond that I'm not sure it can do anything? Maybe everyone here is experiencing a separate issue though, I'm not sure.
@nonagonal we're experiencing a different issue.
I met the same issue and annoying of the ugly gap.
Meanwhile, iTerm2 have a setting of windows of this:
Terminal windows resize smoothy.
Disables snapping to character grid. Holding Control with temporarily disable snap-to-grid.
When you set it to yes, it will be maximize of screen. But I doubt why Terminal.app has no such setting.
@Keleir The same workaround was also metioned here https://github.com/eczarny/spectacle/issues/280#issuecomment-236540753
Most helpful comment
I met the same issue and annoying of the ugly gap.
Meanwhile, iTerm2 have a setting of windows of this:
When you set it to
yes, it will be maximize of screen. But I doubt why Terminal.app has no such setting.