riddle - This party ends in tragedy (I planned it from the start, you see)

The DJ from Moscow was pumping out tunes.

The house mostly bare but for furniture, strewn;
A sofa for three and an armchair for one.
The guests started piling in, looking for fun...

'Twas Elle who came first, seeking somewhere to slouch,
She leaned on the wall with her feet on the couch.
Her boyfriend soon followed and sat down beside,
And cozied in close as they both turned and eyed...
...a blue jay that fluttered in, down from the sky.
More falling than flying - it wasn't clear why.
It flipped as it fell, landing hard with a whack,

Between the two seats and lay dead on its back.

The next to arrive, a conservative man,
With laces as straight as the beeline he ran.
He sat down, oblivious, square on the bird!
And that is where things went from weird to absurd...

...

The twins came next; first one then two,
Like lightning bolts struck from the blue.
The first crashed on the floor, out wide,
With legs propped up, on armchair's side.

When number two came dropping in,
He settled on his brother's shin!
His feet, in turn, then came to lay,
Upon the man, upon the jay!

By now the room was one big din,
But people kept sardining in.
As case in point, there came a gent,
Who, dressed as Mr T, then went,
And crammed himself beside Elle's guy,
And gave high fives to those nearby.

Then Elle rushed in-- Can that be who!?
No time to check... (perhaps there's two?)
She looked for space, dived head-first in,
And made a seat of piled up twin.

But now the time had come at last,
As I stepped into chaos, vast.
I surveyed everything in sight,
Then moved with purpose for the right.

This party ends in tragedy.
I planned it from the start, you see.

'Cos as I sat, they could not flee...
As all that was just ceased to be!

Draw a picture of the room, including everyone's exact position, as it was in the moments before I sat down.

Answer

As pointed out in the comments by crcroberts, this particular room is

a Tetris field.

Final picture:

---

Full explanation:

The DJ from Moscow was pumping out tunes.
The house mostly bare but for furniture, strewn;
A sofa for three and an armchair for one.
The guests started piling in, looking for fun...

Tetris's tune is quite famous, and it is actually a Russian folk song (thanks Alconja!). At the start, the board is empty except for a group of 3 blocks (couch) and a group of one (armchair), probably leftovers from a previous line clear.

'Twas Elle who came first, seeking somewhere to slouch,
She leaned on the wall with her feet on the couch.

Elle is the L piece, which lies close to the wall, on top of the couch.

Her boyfriend soon followed and sat down beside,

And cozied in close as they both turned and eyed...

Elle's boyfriend is an S, who can fit next to an L piece.

...a blue jay that fluttered in, down from the sky.
More falling than flying - it wasn't clear why.
It flipped as it fell, landing hard with a whack,
Between the two seats and lay dead on its back.

The jay is, of course, the J piece, that also happens to be actually blue in the games. It lay on the floor on its back, between the group of 3 and the group of 1.

The next to arrive, a conservative man,
With laces as straight as the beeline he ran.
He sat down, oblivious, square on the bird!
And that is where things went from weird to absurd...

Conservative man can be another term for "square", which is also actually literally mentioned in "square on the bird". So this has to be the O piece.

The twins came next; first one then two,
Like lightning bolts struck from the blue.
The first crashed on the floor, out wide,
With legs propped up, on armchair's side.
When number two came dropping in,
He settled on his brother's shin!
His feet, in turn, then came to lay,
Upon the man, upon the jay!

The twins are two equal pieces, more specifically the "lightning bolt" pieces, aka the Z tetrominoes. One falls on top of the armchair and the second one falls on top of the first one, with one end on top of the square block.

By now the room was one big din,
But people kept sardining in.
As case in point, there came a gent,
Who, dressed as Mr T, then went,
And crammed himself beside Elle's guy,
And gave high fives to those nearby.

Mr. T is the T tetromino, who is adjacent to Elle's boyfriend and also a bunch of others.

Then Elle rushed in-- Can that be who!?
No time to check... (perhaps there's two?)
She looked for space, dived head-first in,
And made a seat of piled up twin.

Another L, fitting in the space that was left for it on the right side.

But now the time had come at last,
As I stepped into chaos, vast.
I surveyed everything in sight,
Then moved with purpose for the right.

This party ends in tragedy.
I planned it from the start, you see.
'Cos as I sat, they could not flee...
As all that was just ceased to be!

"I" is the I tetromino. Once it settles down on the right column, a Tetris will be formed and all four lines will be gone.