refactor(grid): make screen rendering more multibyte than ever before

Problem: buffer text with composing chars are converted from UTF-8
to an array of up to seven UTF-32 values and then converted back
to UTF-8 strings.

Solution: Convert buffer text directly to UTF-8 based schar_T values.

The limit of the text size is now in schar_T bytes, which is currently
31+1 but easily could be raised as it no longer multiplies the size
of the entire screen grid when not used, the full size is only required
for temporary scratch buffers.

Also does some general cleanup to win_line text handling, which was
unnecessarily complicated due to multibyte rendering being an "opt-in"
feature long ago. Nowadays, a char is just a char, regardless if it consists
of one ASCII byte or multiple bytes.

1 files changed, 30 insertions, 0 deletions

diff --git a/test/functional/ui/multibyte_spec.lua b/test/functional/ui/multibyte_spec.lua
index 077dd1a779..d72bf27d6b 100644
--- a/test/functional/ui/multibyte_spec.lua
+++ b/test/functional/ui/multibyte_spec.lua
@@ -228,6 +228,36 @@ describe("multibyte rendering", function()
     ]]}
 
   end)
+
+  it('works with arabicshape and multiple composing chars', function()
+    -- this tests an important edge case: arabicshape might increase the byte size of the base
+    -- character in a way so that the last composing char no longer fits. use "g8" on the text
+    -- to observe what is happening (the final E1 80 B7 gets deleted with 'arabicshape')
+    -- If we would increase the schar_t size, say from 32 to 64 bytes, we need to extend the
+    -- test text with even more zalgo energy to still touch this edge case.
+
+    meths.buf_set_lines(0,0,-1,true, {"سلام့̀́̂̃̄̅̆̇̈̉̊̋̌"})
+    command('set noarabicshape')
+
+    screen:expect{grid=[[
+      ^سلام့̀́̂̃̄̅̆̇̈̉̊̋̌                                                        |
+      {1:~                                                           }|
+      {1:~                                                           }|
+      {1:~                                                           }|
+      {1:~                                                           }|
+                                                                  |
+    ]]}
+
+    command('set arabicshape')
+    screen:expect{grid=[[
+      ^ﺱﻼﻣ̀́̂̃̄̅̆̇̈̉̊̋̌                                                         |
+      {1:~                                                           }|
+      {1:~                                                           }|
+      {1:~                                                           }|
+      {1:~                                                           }|
+                                                                  |
+    ]]}
+  end)
 end)
 
 describe('multibyte rendering: statusline', function()