fix: improve buffer handling in diff view

Use vim.api.nvim_win_call to properly handle buffer and window context
when enabling and disabling diff mode. This ensures commands are run
in the correct window context rather than affecting the current one,
preventing unexpected side effects.

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>

Author: deathbeam

lua/CopilotChat/config/mappings.lua

@@ -377,16 +377,19 @@ return {
         opts.text = table.concat(modified, '\n')
 
         opts.on_show = function()
-          vim.cmd('diffthis')
-          vim.api.nvim_set_current_win(vim.fn.bufwinid(diff.bufnr))
-          vim.api.nvim_win_set_cursor(0, { diff.start_line, 0 })
-          vim.cmd('diffthis')
-          vim.api.nvim_set_current_win(copilot.chat.winnr)
-          vim.api.nvim_win_set_cursor(copilot.chat.winnr, { diff.start_line, 0 })
+          vim.api.nvim_win_call(vim.fn.bufwinid(diff.bufnr), function()
+            vim.cmd('diffthis')
+          end)
+
+          vim.api.nvim_win_call(copilot.chat.winnr, function()
+            vim.cmd('diffthis')
+          end)
         end
 
         opts.on_hide = function()
-          vim.cmd('diffoff')
+          vim.api.nvim_win_call(copilot.chat.winnr, function()
+            vim.cmd('diffoff')
+          end)
         end
       else
         opts.text = tostring(vim.diff(diff.reference, diff.change, {