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Accelerated backspace measures the wrong code point and can split an emoji

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
65/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Quiet
Tech stack
android, java
Domain
mobile

Research direction

Read app/src/main/java/helium314/keyboard/latin/inputlogic/InputLogic.java around lines 2019–2026, starting with the accelerated backspace logic and how it reads the code point before the cursor. Check the deletion length for both a plain character followed by an emoji and an emoji followed by a plain character; done means the second deletion uses the re-read code point and does not split an emoji or over-delete a plain character.

Written by the indexing model from the issue text.

Description

Summary

In InputLogic.handleBackspaceEvent, the accelerated (held-backspace) second deletion measures its length from the first code point instead of the one it is actually about to delete. When the second character back is a multi-code-point emoji, this deletes the wrong number of chars and can split the emoji, leaving an orphaned surrogate or a stray variation selector / ZWJ.

Present at tag v4.1.2 (f0ff166ae).

Location

app/src/main/java/helium314/keyboard/latin/inputlogic/InputLogic.java, around lines 2019–2026:

final int codePointBeforeCursorToDeleteAgain = mConnection.getCodePointBeforeCursor();
if (codePointBeforeCursorToDeleteAgain != Constants.NOT_A_CODE) {
    final int lengthToDeleteAgain = codePointBeforeCursor > 0xFE00          // <-- first code point
            || StringUtils.mightBeEmoji(codePointBeforeCursor)              // <-- first code point
                    ? mConnection.getCharCountToDeleteBeforeCursor()
                    : 1;
    mConnection.deleteTextBeforeCursor(lengthToDeleteAgain);
    totalDeletedLength += lengthToDeleteAgain;
}

The guard correctly re-reads the cursor into codePointBeforeCursorToDeleteAgain, but the ternary that decides the delete length still tests codePointBeforeCursor — the value from before the first deletion.

Consequences

Both directions are wrong:

  • First char plain, second an emoji → the branch evaluates false, so lengthToDeleteAgain is 1. A single char is removed from a multi-char emoji, splitting it.
  • First char an emoji, second plain → the branch evaluates true, so getCharCountToDeleteBeforeCursor() is used against a plain character and may remove more than intended.

Only reachable once mDeleteCount > Constants.DELETE_ACCELERATE_AT, i.e. while holding backspace, which is probably why it has gone unnoticed.

Suggested fix

Use the re-read value in the condition, so it describes the character actually being deleted:

final int lengthToDeleteAgain = codePointBeforeCursorToDeleteAgain > 0xFE00
        || StringUtils.mightBeEmoji(codePointBeforeCursorToDeleteAgain)
                ? mConnection.getCharCountToDeleteBeforeCursor()
                : 1;

Notes

Found while merging v4.1.2 into a downstream fork. This almost certainly predates LeanType — the surrounding accelerated-delete code comes from the HeliBoard/AOSP lineage — so it may be worth forwarding to Helium314/HeliBoard as well.

I have no reproduction on a physical device; this is from reading the code, so please sanity-check the emoji-splitting claim against real input before acting on it.

Dominant language
Kotlin
Stars
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Forks
59
Avg merge
14h 37m
Merged PRs (30d)
54

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