Fix crash when reading non-UTF16 files

[amirkiarafiei]

Problem

1. The application crashes with a segmentation fault when a user opens an ASCII or UTF-8 text file.
2. UTF-16 files larger than 4MB are truncated on load (UTF-16 files larger than 4MB are silently truncated on load (regression from #1139) #1142).

Cause

Commit a74152a added UTF-16 support. It also introduced an unconditional call to decoder(input) in src/ScintillaNext.cpp.

When the file is not UTF-16, decoder is not initialized. Invoking an uninitialized QStringDecoder on Qt 6 dereferences a null pointer.

Additionally, decoder was declared inside the chunk loop. On files larger than 4MB, decoder was re-instantiated on every chunk.

Solution

1. Remove the unconditional call to decoder(input) outside the UTF-16 conditional block.
2. Move QStringDecoder decoder before the chunk loop to maintain decoder state across chunks.

Fixes #1142

Note

This change is applied by the AI Gemini 3.8 Flash of Antigravity-CLI harness and the responsibility is for the author (Amirkia RAFIEI OSKOOEI).

[amirkiarafiei]

Hello.
I am the author of the commit. The fix was applied with the help of Code Agents, based on the project's architecture and conventions.

[amirkiarafiei]

I have updated the PR to pair QStringDecoder with a persistent QStringEncoder(QStringEncoder::Utf8).

Because chunk 1 subtracts the 2-byte BOM, it ends on an offset boundary (4194302 % 4 == 2) that can split a 4-byte UTF-16 surrogate pair across the 4MB chunk boundary. Stateless toUtf8() on chunk 1 would turn an unmatched high surrogate into replacement characters, whereas a persistent QStringEncoder buffers and carries the pending surrogate into the next chunk seamlessly.

[dail8859]

Appreciate the help with this and identifying these potentially nasty issues :)

I pushed a small tweak to the code. It goes ahead and instantiates the encoder. I would foresee the overhead of creating the encoder even when not needed would be miniscule compared to the rest of the disk read, encoding, and processing by Scintilla.