fix(terminal): handle split composing chars at right edge (#37694)

Problem:
Recombining composing chars in terminal doesn't work at right edge.

Solution:
Check for the case where printing the previous char didn't advance the
cursor. Reset at_phantom when returning to combine_pos.

src/nvim/vterm/state.c

@@ -342,13 +342,15 @@ static int on_text(const char bytes[], size_t len, void *user)
 
   // See if the cursor has moved since
   if (state->pos.row == state->combine_pos.row
-      && state->pos.col == state->combine_pos.col + state->combine_width) {
+      && state->pos.col >= state->combine_pos.col
+      && state->pos.col <= state->combine_pos.col + state->combine_width) {
     // This is a combining char. that needs to be merged with the previous glyph output
     if (utf_iscomposing((int)state->grapheme_last, (int)codepoints[i], &state->grapheme_state)) {
       // Find where we need to append these combining chars
       grapheme_len = state->grapheme_len;
       grapheme_state = state->grapheme_state;
       state->pos.col = state->combine_pos.col;
+      state->at_phantom = 0;
       recombine = true;
     } else {
       DEBUG_LOG("libvterm: TODO: Skip over split char+combining\n");

test/functional/terminal/buffer_spec.lua

@@ -758,7 +758,7 @@ describe(':terminal buffer', function()
   end)
 
   it('handles split UTF-8 sequences #16245', function()
-    local screen = Screen.new(50, 7)
+    local screen = Screen.new(50, 10)
     fn.jobstart({ testprg('shell-test'), 'UTF-8' }, { term = true })
     screen:expect([[
       ^å                                                 |
@@ -766,7 +766,24 @@ describe(':terminal buffer', function()
       1: å̲                                              |
       2: å̲                                              |
       3: å̲                                              |
-                                                        |*2
+                                                        |
+      [Process exited 0]                                |
+                                                        |*3
+    ]])
+    -- Test with Unicode char at right edge using a 4-wide terminal
+    command('bwipe! | set laststatus=0 | 4vnew')
+    fn.jobstart({ testprg('shell-test'), 'UTF-8' }, { term = true })
+    screen:expect([[
+      ^å   │                                             |
+      ref:│{1:~                                            }|
+       å̲  │{1:~                                            }|
+      1: å̲│{1:~                                            }|
+      2: å̲│{1:~                                            }|
+      3: å̲│{1:~                                            }|
+          │{1:~                                            }|
+      [Pro│{1:~                                            }|
+      cess│{1:~                                            }|
+                                                        |
     ]])
   end)
 

test/unit/vterm_spec.lua

@@ -992,6 +992,7 @@ describe('vterm', function()
     push('e' .. string.rep('\xCC\x81', 20), vt)
     expect('putglyph 65,301,301,301,301,301,301,301,301,301,301,301,301,301,301 1 0,0') -- and nothing more
 
+    -- Combining across buffers multiple times
     reset(state, nil)
     push('e', vt)
     expect('putglyph 65 1 0,0')
@@ -1000,6 +1001,32 @@ describe('vterm', function()
     push('\xCC\x82', vt)
     expect('putglyph 65,301,302 1 0,0')
 
+    -- Combining across buffers at right edge
+    reset(state, nil)
+    push('\x1b[5;80H', vt)
+    push('e', vt)
+    expect('putglyph 65 1 4,79')
+    push('\xCC\x81', vt)
+    expect('putglyph 65,301 1 4,79')
+    push('\xCC\x82Z', vt)
+    expect('putglyph 65,301,302 1 4,79\nputglyph 5a 1 5,0')
+
+    -- Combining regional indicators
+    reset(state, nil)
+    push('\x1b[5;77H', vt)
+    push('🇦', vt)
+    expect('putglyph 1f1e6 2 4,76')
+    push('🇩', vt)
+    expect('putglyph 1f1e6,1f1e9 2 4,76')
+    push('🇱', vt)
+    expect('putglyph 1f1f1 2 4,78')
+    push('🇮', vt)
+    expect('putglyph 1f1f1,1f1ee 2 4,78')
+    push('🇲', vt)
+    expect('putglyph 1f1f2 2 5,0')
+    push('🇨', vt)
+    expect('putglyph 1f1f2,1f1e8 2 5,0')
+
     -- emoji with ZWJ and variant selectors, as one chunk
     reset(state, nil)
     push('🏳️‍🌈🏳️‍⚧️🏴‍☠️', vt)