lsp: log more efficiently

* Do not log when in visual mode, select mode, or while waiting on
  completion. This keeps the screen from being cleared or from
  generating an exception when logging would otherwise change to the log
  window to scroll.
* Do not change to the log window merely to write to it. The only
  operation that is now required to change to the log window is to
  scroll it, so limit the time that it's the active window to that
  single operation.

autoload/go/lsp.vim

@@ -1332,6 +1332,15 @@ let s:logtimer = 0
 function! s:debugasync(timer) abort
   let s:logtimer = 0
 
+  " set the timer to try again if Vim is in a state where we don't want to
+  " change the window.
+  let l:state = state('a')
+  let l:mode = mode(1)
+  if len(l:state) > 0 || l:mode[0] == 'v' || l:mode[0] == 'V' || l:mode[0] == 's' || l:mode =~ 'CTRL-V'
+    let s:logtimer = timer_start(go#config#DebugLogDelay(), function('s:debugasync', []))
+    return
+  endif
+
   if !go#util#HasDebug('lsp')
     let s:log = []
     return
@@ -1347,30 +1356,35 @@ function! s:debugasync(timer) abort
       silent file `='__GOLSP_LOG__'`
       setlocal buftype=nofile bufhidden=wipe nomodified nobuflisted noswapfile nowrap nonumber nocursorline
       setlocal filetype=golsplog
-    else
-      call win_gotoid(l:log_winid)
+      call win_gotoid(l:winid)
     endif
 
+    let l:logwinid = bufwinid(l:name)
+
     try
-      setlocal modifiable
+      call setbufvar(l:name, '&modifiable', 1)
       for [l:event, l:data] in s:log
         call remove(s:log, 0)
-        if getline(1) == ''
-          call setline('$', printf('===== %s =====', l:event))
+        if getbufline(l:name, 1)[0] == ''
+          call setbufline(l:name, '$', printf('===== %s =====', l:event))
         else
-          call append('$', printf('===== %s =====', l:event))
+          call appendbufline(l:name, '$', printf('===== %s =====', l:event))
         endif
-        call append('$', split(l:data, "\r\n"))
+        call appendbufline(l:name, '$', split(l:data, "\r\n"))
       endfor
+
+      " TODO(bc): how to move the window's cursor position without switching
+      " to the window?
+      call win_gotoid(l:logwinid)
       normal! G
-      setlocal nomodifiable
-    finally
       call win_gotoid(l:winid)
+      call setbufvar(l:name, '&modifiable', 0)
+    finally
     endtry
   catch
-    call go#util#EchoError(v:exception)
+    call go#util#EchoError(printf('at %s: %s', v:throwpoint, v:exception))
   finally
-    " retry in when there's an exception. This can happen when trying to do
+    " retry when there's an exception. This can happen when trying to do
     " completion, because the window can not be changed while completion is in
     " progress.
     if len(s:log) != 0