fix(chat): correct header highlighting for multi-byte characters (#1385)

The previous implementation used vim.fn.strwidth() to calculate the end column for header highlighting. This function returns the display width, which can differ from the byte length when multi-byte characters like emojis are present.

Since the end_col for extmarks expects a byte-based index, this caused the highlighting to be applied incorrectly.

This patch corrects the issue by using the byte length (#header_value) for the end_col of the highlight extmark, while still using vim.fn.strwidth() for virt_text_win_col to ensure correct visual alignment of UI elements.

Fixes #1384

Author: ctchen222

lua/CopilotChat/ui/chat.lua

@@ -620,21 +620,21 @@ function Chat:render()
       if id then
         -- Draw the separator as virtual text over the header line, hiding the id and anything after the header
         if self.config.highlight_headers then
-          local sep_col = vim.fn.strwidth(header_value)
-          vim.api.nvim_buf_set_extmark(self.bufnr, highlight_ns, l - 1, sep_col, {
+          local header_width = vim.fn.strwidth(header_value)
+          vim.api.nvim_buf_set_extmark(self.bufnr, highlight_ns, l - 1, 0, {
+            end_col = #header_value,
+            hl_group = 'CopilotChatHeader',
+            priority = 100,
+            strict = false,
+          })
+          vim.api.nvim_buf_set_extmark(self.bufnr, highlight_ns, l - 1, #header_value, {
             virt_text = {
               { string.rep(self.separator, vim.go.columns), 'CopilotChatSeparator' },
             },
-            virt_text_win_col = sep_col,
+            virt_text_win_col = header_width,
             priority = 200,
             strict = false,
           })
-          vim.api.nvim_buf_set_extmark(self.bufnr, highlight_ns, l - 1, 0, {
-            end_col = sep_col,
-            hl_group = 'CopilotChatHeader',
-            priority = 100,
-            strict = false,
-          })
         end
 
         -- Finish previous message