Problem summary

Under the following conditions, Vim does not update the cells of unicode ambiguous-width character properly.

• On a terminal which treats ambiguous-width as 1 cell
• :set ambiwidth=double

Repro steps

Create 1.txt and 2.txt

vim --clean

:e 1.txt
:normal! izzzzzz
:w
:e 2.txt
:exe "normal!" "iab\u03bbef"
:wq

• \u03bb is λ (lambda)

1.txt:

2.txt: (ambw=double)

Open 1.txt and 2.txt

vim --clean +'set ambw=double'

:e 1.txt
:e 2.txt

Result

z of 1.txt remains just behind lambda of 2.txt.

Details

When drawing a content of 2.txt, Vim outputs into terminal:

>>>   CSI 1;1H                CUP / move cursor to (row=1, col=1)
>>>  abλ
>>>   CSI 1;5H                CUP / move cursor to (row=1, col=5)
>>>  ef

Vim treats lambda as 2 cells but terminal treats it as 1 cell, so (row=1, col=4) cell is not updated.

Solution

• Overwrite the cells by 2-spaces before output ambiguous-width (when ambw=double)