Some scripts, among them Arabic, Hebrew, and most scripts of India and SE Asia, are written as combinations of consonants and vowel marks that combine with them.
In most text editors and word processors, when the cursor is after a combination of a consonant and a vowel, and the backspace key s pressed, the vowel is deleted first and the the consonant.
For example if you have the Devanagari combination गा (ग [g] + ा [a]), these are two Unicode characters, which the font joins automatically. If the cursor is after them and you press the backspace key, then the second character ( ा) is supposed to be deleted, and only then the first (ग). That is what happens in most text editors, including MediaWiki's source editor.
In the VisualEditor, backspace immediately deletes the whole cluster. This behavior is unexpected for most users.
To complicate things, when the cursor is before the combined character and the Delete key is pressed, the expected behavior is to delete the whole cluster. This is what happens in the VisualEditor now, and this must be kept like that. For cursor movement, back and forth, the cluster must also be treated as one character, so if the cursor is before गा and the right-pointing arrow is pressed, the cursor is supposed to immediately go after the गा. This also works correctly now, and must be kept.