I think your choice of license may be incompatible with the GitHub terms of service--there's an implied right to fork by using GitHub, but your license states no derivatives are allowed. IANAL, but something to look into: https://help.github.com/articles/open-source-licensing
As I mentioned in another comment, it's pretty slow under Firefox.
Lastly, the game is quite pretty, but I feel that since the gameplay involved is so precise, the imprecise visuals can be confusing. It's hard to tell exactly when a piece is going to set in place, when a group is eliminated, what would happen if you rotate and a piece is blocked by a stack already in place, etc. I went by pure geometry at first, but I definitely got bitten a few times where a piece moved after I thought it was in place, or a piece didn't get eliminated with the rest of a group since it hadn't fully landed in its column. The collision animations don't quite help there either.
"By setting your repositories to be viewed publicly, you agree to allow others to view and fork your repositories."
However, there's no further mention of the word "fork" in the T&Cs. Simply forking a repository does not imply that a derivative work has been created. A fork is simply a verbatim copy.
To create a derivative work, someone would need to both fork and then commit a change back to that fork. Of course, it's sort of implied through common (imprecise) use of the word that this is what it's intended to mean, but it's not actually what it says.
While I'm not a lawyer, I believe that a more precisely-worded licence in the repository would take precedence over the ambiguously worded T&Cs. However, Github could at any time clarify the T&Cs to more clearly state that forking does imply creation of derivatives. And as with most online services, if the terms do get changed, "Continued use of the Service after any such changes shall constitute your consent to such changes."
I've been bitten in the past when a user deleted their account or changed their name and my references broke.
I actually had an opposite problem, the game was incredibly generous with its imprecise collision. I could rotate twice and catch two rectangles falling at the same time on the same paddle and finish a group. I was overwhelmed by the number of sides I had to simultaneously defend and the number of different falling pieces. The animations, I don't think, made much difference in difficulty or ease. Mostly I wound up surprised that I had cleared a group, rather than the opposite problem you described.
Funny we both had very opposite experiences.
Kind of misleading to use the name "Hextris" for what looks like a very different game. The original Hextris is 2D falling blocks exactly like Tetris, except that the world consists of hexagons, and so the pieces have six rotations. I played this like crazy for a brief time in the mid 1990's.
Remembered it recently and based my own game on hexagons!
I'm struggling to beat 5863
I tried again in Chrome and its a lot faster, and does get faster throughout the game as well. More fun and challenging, but yeah--it does get too fast too quickly.
Btw the speed seems to depend on the machine you play it on. On my wife's close to 10 year old laptop it's way slower than on mine. But the rotation is still fast enough, so scores are higher. She's at 20000 now and the speed is roughly similar to what I get at 1000 points. Old hardware FTW.
One quick criticism:
I played a couple times and maxed out around 700pts. Then played a game where I didn't touch a single key -- no rotation whatsoever -- and managed to score 3292pts.
I'm not sure what this indicates, but it feels off.
Maybe more colors could be added? Game balance has been really tricky with this game, as changes play out very differently on mobile devices and "real computers."
From a cursory look (my math may be a little off) it seems like the odds of a tile matching at least one of its 4 cardinal-direction neighbors is ~57% (20 of 35 order-independent variations), and the odds of a tile matching 2 or more of its randomly selected neighbors is ~28%. In other words, with just random play you're going to get a match out of every `+` shaped collection of 5 tiles nearly 1/3rd of the time. Factoring the three-in-a row possibility the odds of a match will go up from there. (And since a match will eliminate several tiles of given color, the odds of getting a match on the "chain reaction" must also be quite high.)
3d version: http://theodor.lauppert.ws/games/frac.htm
4d version: https://www.youtube.com/watch?v=Tzyws5ZkSYg
One feature that might be cool to have would be to make the gray background hexagon one block larger if you get four or five blocks in a row. That way, you'll be able to keep your ahead above water a little better when things really start to speed up. It would add another level of strategy and planning to the game.
Keep up the great work!
Having constant interval timing in your game engine is an absolute must have.
A few comments:
I think a key to drop faster would be a nice addition.
The speed progression could be tweaked a bit. It's a bit boring until it gets faster, and it takes a while.
I believe that game was made for a Ludum Dare originally.
Can't stop playing regardless.
On the side note : The background music in the trailer seems heavily inspired from the mission impossible theme :)
Maybe a little walk through/instructions displayed for longer. Had to start over to quickly read again (was zoning out first time)
Also particularly impressive as the creator(s) seem to be high school students.
This would allow combos even if the pile heights are different.
It's the perfect mashup of Tetris and Super Hexagon.
Marco's new app, Overcast also has UI drawn almost entirely with code (all icons are generated with PaintCode ), and his app  is 4.5mb.