fix(variables): consistent expand behaviour with vscode

Adapters can return different IDs for the same frame across steps so we
can't use frame IDs to keep state.
VSCode doesn't distinguish variable names across frames so this change
is consistent with VSCode's behaviour

See #230

lua/dapui/components/variables.lua

@@ -24,30 +24,23 @@ return function(client, send_ready)
   ---@type table<string, string>
   local rendered_vars = {}
 
-  local function reference_prefix(frame_id, path, variable)
+  local function reference_prefix(path, variable)
     if variable.variablesReference == 0 then
       return " "
     end
-    return config.icons[expanded_children[frame_id][path] and "expanded" or "collapsed"]
+    return config.icons[expanded_children[path] and "expanded" or "collapsed"]
   end
 
   ---@param path string
-  local function path_changed(frame_id, path, value)
-    return rendered_vars[frame_id][path] and rendered_vars[frame_id][path] ~= value
+  local function path_changed(path, value)
+    return rendered_vars[path] and rendered_vars[path] ~= value
   end
 
   ---@param canvas dapui.Canvas
   ---@param parent_path string
   ---@param parent_ref integer
   ---@param indent integer
   local function render(canvas, parent_path, parent_ref, indent)
-    local frame_id = client.session.current_frame and client.session.current_frame.id
-    if not frame_id then
-      return
-    end
-    expanded_children[frame_id] = expanded_children[frame_id] or {}
-    rendered_vars[frame_id] = rendered_vars[frame_id] or {}
-
     if not canvas.prompt and prompt_func then
       canvas:set_prompt("> ", prompt_func, { fill = prompt_fill })
     end
@@ -59,7 +52,7 @@ return function(client, send_ready)
 
       canvas:write({
         string.rep(" ", indent),
-        { reference_prefix(frame_id, var_path, variable), group = "DapUIDecoration" },
+        { reference_prefix(var_path, variable), group = "DapUIDecoration" },
         " ",
         { variable.name, group = "DapUIVariable" },
       })
@@ -70,16 +63,16 @@ return function(client, send_ready)
       end
 
       local var_group
-      if path_changed(frame_id, var_path, variable.value) then
+      if path_changed(var_path, variable.value) then
         var_group = "DapUIModifiedValue"
       else
         var_group = "DapUIValue"
       end
-      rendered_vars[frame_id][var_path] = variable.value
+      rendered_vars[var_path] = variable.value
       local function add_var_line(line)
         if variable.variablesReference > 0 then
           canvas:add_mapping("expand", function()
-            expanded_children[frame_id][var_path] = not expanded_children[frame_id][var_path]
+            expanded_children[var_path] = not expanded_children[var_path]
             send_ready()
           end)
           if variable.evaluateName then
@@ -113,7 +106,7 @@ return function(client, send_ready)
         add_var_line(variable.value)
       end
 
-      if expanded_children[frame_id][var_path] and variable.variablesReference ~= 0 then
+      if expanded_children[var_path] and variable.variablesReference ~= 0 then
         render(canvas, var_path, variable.variablesReference, indent + config.render.indent)
       end
     end