Same as other commenters here. The only input that appeared to work is Left mouse.
Here's what I learned messing with the game so far. You can only short or long tap the triangle in the middle. Both create a pulse: the longer one has a bigger radius.
There's a white bar at the top that shows you how long you have until you have to click the triangle before you lose.
PURE SPOILERS FROM HERE ON OUT
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If that bar ticks down to nothing: the game ends, and you get a black screen with "[Signal Lost] Structural Failure. Silence"
If you hold out long enough: the game also ends, and you get "[System Error] Causal Loop Severed. ASCENSION"
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In the beginning of the game, you get these messages randomly.
"[Coordinates Lost] Life support MANUAL Transmit"
"Your 3D structure tears"
"I observe"
"Why resist entropy"
I think it's just the thing that's referred to in this game's LD page description, reaching out to you.
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The messages you get under the triangle are based on what you pressed, and if you haven't pressed for awhile, random messages like the above. Short taps give "Weak Peturbation" and "Insufficient" - Long taps give "Coherent Wave" and "Acceptable Feed". Although these changes based on where you're at in the game (described in the next section)
3 short taps in a row gives "[PARSE] Panic Logic"
3 long taps in a row gives "[OVERLOAD] Willpower Recognized"
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At 2 points in the game, "events" happen. One at T-30 and one at T-60. The game ends at T-90. The first is
[WARNING] Gravity Well Approaching.
The second is
[CRIT] Event horizon breached. Physics obsolete.
After the first, the messages you get while waiting change, something like...
"Parse the despair"
"Light Cannot Escape"
"Mass Leaking. Give it!"
Not sure if some kind of hints, or just flavor text.
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In the bottom right, there's a track of what pulses you most recently did. Short taps are "S", Long taps (no matter how long) are "L". So if it says something like "SLLL" your last for were short Long Long Long. Sometimes it resets, other times, it adds to the current chain. I don't see any way to have it go longer than 4 characters, and I have seen it reset after 3.
I'm guessing you're sending out Morse Code. I don't know what you'd have to send, or how to have a letter like "e" without it thinking its something else. But occasionally you get a "Syntax Learned" message, only after a successful morse code letter.
And obviously, as stated above with the 3 short taps and 3 long taps, "S" and "O" have a custom message.
I tried all the letters, and these were the only ones I found to be recognized
S - Short short short - "[PARSE] Panic Logic"
O- Long Long Long - "[OVERLOAD] Willpower Recognized"
X - Long Short Short Long - "[RES] Syntax Learned"
U - Short Short Long - "[RES] Syntax Learned"
R - Short Long Short - "[DECODE] No rescue here"
C - Long Short Long Short - "[DECODE] No rescue here"
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If there's a third end-game condition, I have yet to figure out what inputs are required.
Any thoughts? @ me with em, I'd love to hear it! ... Even if by this point... I'm pretty sure there's no hidden ending....