fix: prevent infinite detach/re-attach loop on non-buflisted buffers

Fixes #576

Author: AntoineGS

lua/copilot/client/init.lua

@@ -184,7 +184,7 @@ local function on_buf_enter(bufnr)
     -- This is to handle the case where the filetype changes after the buffer is already attached,
     -- causing the LSP to raise an error
     local previous_ft = util.get_buffer_previous_ft(bufnr)
-    if previous_ft and (previous_ft ~= vim.bo[bufnr].filetype) then
+    if previous_ft and vim.bo[bufnr].buflisted and (previous_ft ~= vim.bo[bufnr].filetype) then
       logger.trace("filetype changed, detaching and re-attaching")
       M.buf_detach_if_attached(bufnr)
     end

tests/test_client.lua

@@ -436,4 +436,47 @@ T["client()"]["disabled copilot does not spam warnings on buffer enter"] = funct
   MiniTest.expect.equality(count, 0)
 end
 
+T["client()"]["on_buf_enter skips filetype check for non-buflisted buffers"] = function()
+  child.configure_copilot()
+
+  -- Create a non-buflisted buffer with a filetype mismatch and trigger BufEnter.
+  -- This simulates floating windows / preview buffers that should not trigger
+  -- the filetype-change detach+re-attach cycle.
+  child.lua([[
+    local util = require("copilot.util")
+
+    local buf = vim.api.nvim_create_buf(false, true)  -- nobuflisted, scratch
+    vim.api.nvim_set_option_value("filetype", "lua", { buf = buf })
+
+    -- Manually set previous_ft to simulate a buffer that was previously attached.
+    -- buf_attach won't succeed for non-buflisted buffers (should_attach rejects them),
+    -- so previous_ft wouldn't normally be set. In the real bug scenario, this state
+    -- can be reached through various pathways.
+    util.set_buffer_previous_ft(buf, "lua")
+
+    -- Change filetype on same non-buflisted buffer
+    vim.api.nvim_set_option_value("filetype", "python", { buf = buf })
+
+    -- Trigger BufEnter (simulating entering the buffer)
+    vim.api.nvim_exec_autocmds("BufEnter", { buffer = buf })
+
+    -- Wait for scheduled callbacks to execute
+    vim.wait(200, function() return false end, 10)
+  ]])
+
+  -- The filetype change should NOT cause detach+re-attach for non-buflisted buffers
+  local detach_was_called = child.lua([[
+    local log_content = ""
+    local logfile = io.open("./tests/logs/test_client.log", "r")
+    if logfile then
+      log_content = logfile:read("*a")
+      logfile:close()
+    end
+    -- If the guard works, we should NOT see "filetype changed" for non-buflisted buffers
+    return log_content:find("filetype changed, detaching and re%-attaching") ~= nil
+  ]])
+
+  MiniTest.expect.equality(detach_was_called, false)
+end
+
 return T