IME Conversion Issue with Japanese Input

[k-fujimaru]

Hello!

I work on localizing the UI into Japanese, I've encountered a problem specific to certain language regions.

The issue occurs when using an Input Method Editor (IME) to type: conversion characters are mistakenly confirmed.

• This seems to happen in languages that use an IME, such as Japanese.
• However, when entering edit mode with the F2 key and then typing, it works correctly.

Expected behavior/appearance (this example is from Google Sheets):

Select a cell -> type "ohayou" -> press Enter -> select another cell -> type "konnnitiha" -> press Enter

https://github.com/gristlabs/grist-core/assets/35284551/69731aed-173e-4e97-b2f9-100ce423b434

Behavior/appearance in Grist:

Select a cell -> type "ohayou" -> press Enter -> select another cell -> type "konnnitiha" -> press Enter

https://github.com/gristlabs/grist-core/assets/35284551/f4d86063-81e4-498c-b160-73e3ac432436

This issue only arises when Japanese input is enabled. It might also occur with Chinese or Korean inputs. I've checked the source code, but I couldn't pinpoint the problem myself. The cause might be related to the handling of the input event or the processing within the NTextEditor.