Feature request: Support overlapping input for Japanese Modifier (Dakuten) key

[word5355]

Use Cases

Background (How it works in Japanese):
In the Japanese 12-Key layout, the "Language Switch" key has a dual purpose. After you type a base character (like か), this key temporarily turns into a "Modifier (Dakuten)" key. Pressing it adds a voicing mark (turning か into が).

The Problem:
FUTO Keyboard's current behavior prevents fast typing because of a delay in this key transformation.
When typing quickly with two thumbs, users naturally press the modifier key before releasing the base character key (overlapping presses). However, FUTO only changes the Language key into the Modifier key after the base character key is released.

The Result:
Because of this delay, if a user taps the modifier key while still holding the base character key down, the keyboard thinks I want to switch languages. This causes unintended language swaps, frequent typing errors, and makes high-speed typing hard.

---

I have attached a screen recording to demonstrate the issue.

1. FUTO Keyboard (Normal typing):
   First, I type か (Ka) and then tap the modifier key after releasing it. It correctly outputs が (Ga).
2. FUTO Keyboard (Overlapping input - The Issue):
   Next, I press and hold か and tap the Language/Modifier key with another finger before releasing the first one.
   Result: The keyboard switches languages instead of adding the voicing mark.
3. Gboard (Comparison - Expected behavior):
   Finally, I switch to Gboard. Even while I am holding down the か key, tapping the modifier key immediately outputs が.
   Note: On Gboard, the icon does not change to the Modifier key icon yet, but the input logic correctly prioritizes the modifier function over the language switch when a base character is active.

screen.mp4

Proposal

I propose changing the trigger for the key state transition. The "Language Switch" key should transform into the "Modifier (Dakuten)" key immediately when a base character is touched, instead of waiting for it to be released.

Current behavior (The Problem):

1. Press and hold か (Ka).
2. While holding か, tap the Language key.
3. Result: The keyboard switches languages (because the key hasn't transformed yet).

Proposed behavior (The Solution):

1. Press and hold か (Ka).
2. The Language key immediately acts as the Modifier key.
3. While holding か, tap the Modifier key.
4. Result: The character is correctly modified to が without switching languages.

References

No response