Fix drawing ambiguous-width character

[ichizok]

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)

[codecov-io]

Codecov Report

Merging #2436 into master will increase coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #2436      +/-   ##
==========================================
+ Coverage   74.51%   74.54%   +0.02%     
==========================================
  Files          92       92              
  Lines      132721   132726       +5     
  Branches    29112    29113       +1     
==========================================
+ Hits        98903    98940      +37     
- Misses      33783    33786       +3     
+ Partials       35        0      -35

Impacted Files	Coverage Δ
src/screen.c	73.87% <66.66%> (-0.03%)	⬇️
src/os_unix.c	54.23% <0%> (-0.14%)	⬇️
src/message.c	68.67% <0%> (-0.05%)	⬇️
src/channel.c	82.72% <0%> (-0.05%)	⬇️
src/window.c	81.77% <0%> (+0.03%)	⬆️
src/gui.c	47.5% <0%> (+0.05%)	⬆️
src/if_py_both.h	76.59% <0%> (+0.14%)	⬆️
src/if_xcmdsrv.c	84.71% <0%> (+0.17%)	⬆️
src/gui_beval.c	62.66% <0%> (+0.42%)	⬆️
src/libvterm/src/screen.c	53.34% <0%> (+6.69%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a703aae...a179328. Read the comment docs.