Dialogue Pause Desync

Specifications

Versions: 1.00 - 1.00a - 1.00b - 1.00c - 1.00d
Difficulty: Easy - Normal - Hard - Lunatic - Extra - ~~Last Word~~
Mode: Main game - Practice mode - ~~Spell Practice~~
Shottype: Border Team - Magic Team - Ghost Team - Scarlet Team - Reimu - Yukari - Marisa - Alice - Youmu - Yuyuko - Sakuya - Remilia

What happens

Pausing during dialogue in-game may result in a desync. The desync may not be noticeable until entering the bossfight.

How it happens

The desync is caused by holding the shoot button Z while the game is exiting out of the pause menu. In particular, the player commonly holds Z when pressing on the "Resume to game" option in the pause menu, i.e. when unpausing.

Why it happens

This desync comes down to how the game transitions from the pause state back into active gameplay while evaluating button events.

In EoSD, PCB, and IN, input handling does not fully freeze when the game is paused. Instead, the game continues updating the internal key state buffer even while gameplay logic is halted. When unpausing, the game resumes comparing the current key state against the immediately previous recorded state to determine whether a press occurred.

Input interpretations

There exist four input interpretations:

Key is held: Active
Key is not held: Inactive
Key is pressed (Rising edge): Inactive -> Active
Key is released (Falling edge): Active -> Inactive

Most common actions are done when a key is held. Examples:

Player shooting
Player moving
Skipping through dialogue (by using Ctrl button)

Some actions are done when a key is pressed. Examples:

Skipping through dialogue (by using Z button)
Player bombing

There are some issues that can arise from when key is pressed, which is explained below:

Live playthrough

Consider the following table of a player pausing mid-game during dialogue:

Global timer	1	2	3	4	5	6	7	8
In-game timer	1	2	3	3	3	3	4	5
Is Z button held?

The Global timer is a value that increases by 1 every frame.
The In-game timer is a value that increases by 1 every frame unless the game is paused.
The Is Z button held? is a value that is either True (marked with ) or False (marked with ).

The three highlighted columns indicate that the game is paused The game is paused in the Global Timer interval [4, 6]

When skipping through dialogue by pressing the Z button, The Z key must be pressed This means that one frame before Z is "", Z must be "". for it to be a Z press.
When the Global timer is equal to 7, the game is no longer paused. The game then checks if Z is pressed. For this, the game checks the following two conditions:

Is Z button held? = on Global timer = 7;
Is Z button held? = on Global timer = 6.

The second condition is not met, so Z is not pressed This means that the dialogue is not skipped while playing the game yourself.

Replay mode

Consider the same table from before:

Global timer	1	2	3	4	5	6	7	8
In-game timer	1	2	3	3	3	3	4	5
Is Z button held?

Now, the game does not take into account the three highlighted columns since those inputs have never been saved into the replay. Because of this, we instead look at the value of In-game timer.

When the In-game timer is equal to 4, the game is no longer paused. The game then checks if Z is pressed. For this, the game checks the following two conditions:

Is Z button held? = on In-game timer = 4;
Is Z button held? = on In-game timer = 3.

All two conditions are met, so Z is pressed on In-game timer = 4, which can lead to the dialogue being skipped only while in replay mode.

Conclusion

In conclusion:

Z is not pressed while playing the game on dialogue.
Z is pressed while watching the replay on dialogue.

The inputs are the same in both cases, but its effect is different due to the fact Z must be pressed. This causes the desync glitch.

Holding Z while unpausing does not progress the dialogue in the live playthrough, but it does progress the dialogue in its replay mode, causing a desync. In the live playthrough, the Z input is seen as a held state but in the replay mode the Z input is seen as a rising edge. This discrepancy causes the game to handle the dialogue skipping differently, thus leading to a desync.