feat(lsp): skip or reduce debounce after idle (#16881)

The idea of the debounce is to avoid overloading a server with didChange
notifications. So far this used a constant value to group changes within
an interval together and send a single notification. A side effect of
this is that when you were idle, notifications are still delayed.

This commit changes the logic to take the time the last notification
happened into consideration, if it has been greater than the debounce
interval, the debouncing is skipped or at least reduced.

runtime/lua/vim/lsp.lua

@@ -306,7 +306,6 @@ local function once(fn)
   end
 end
 
-
 local changetracking = {}
 do
   --@private
@@ -327,6 +326,7 @@ do
     if not state then
       state = {
         pending_changes = {};
+        last_flush = {};
         use_incremental_sync = (
           if_nil(client.config.flags.allow_incremental_sync, true)
           and client.resolved_capabilities.text_document_did_change == protocol.TextDocumentSyncKind.Incremental
@@ -347,9 +347,12 @@ do
   function changetracking.reset_buf(client, bufnr)
     changetracking.flush(client)
     local state = state_by_client[client.id]
-    if state and state.buffers then
+    if state then
+      if state.buffers then
         state.buffers[bufnr] = nil
       end
+      state.last_flush = {}
+    end
   end
 
   ---@private
@@ -361,6 +364,33 @@ do
     end
   end
 
+  ---@private
+  --
+  -- Adjust debounce time by taking time of last didChange notification into
+  -- consideration. If the last didChange happened more than `debounce` time ago,
+  -- debounce can be skipped and otherwise maybe reduced.
+  --
+  -- This turns the debounce into a kind of client rate limiting
+  local function next_debounce(debounce, state, bufnr)
+    if debounce == 0 then
+      return 0
+    end
+    local ns_to_ms = 0.000001
+    local last_flush = state.last_flush[bufnr]
+    if not last_flush then
+      return debounce
+    end
+    local now = uv.hrtime()
+    local ms_since_last_flush = (now - last_flush) * ns_to_ms
+    local remaining_debounce = debounce - ms_since_last_flush
+    if remaining_debounce > 0 then
+      return remaining_debounce
+    else
+      state.last_flush[bufnr] = now
+      return 0
+    end
+  end
+
   ---@private
   function changetracking.prepare(bufnr, firstline, lastline, new_lastline)
     local incremental_changes = function(client)
@@ -383,7 +413,7 @@ do
         return
       end
       local state = state_by_client[client.id]
-      local debounce = client.config.flags.debounce_text_changes or 150
+      local debounce = next_debounce(client.config.flags.debounce_text_changes or 150, state, bufnr)
       if debounce == 0 then
         local changes = state.use_incremental_sync and incremental_changes(client) or full_changes()
         client.notify("textDocument/didChange", {
@@ -406,6 +436,7 @@ do
       end
       state.pending_change = function()
         state.pending_change = nil
+        state.last_flush[bufnr] = uv.hrtime()
         if client.is_stopped() or not vim.api.nvim_buf_is_valid(bufnr) then
           return
         end