Working better now (but see notes)

Even so, some things can get screwy if the component tree sets
suspicious `key`s. Like:

```lua
{
  'String 1',                    -- auto-assigned key = idx(1)
  h(Component, { key = 1 }, {}), -- conflicting key(1)
  'String 2',                    -- auto-assigned key = idx(3)
  h(Component, { key = 2 }, {}), -- key = 2
}
```

In this example, there are two elements competing for key `1`, which
causes weird render issues.

lua/u/renderer.lua

@@ -86,7 +86,7 @@ M.Renderer = Renderer
 ---   component?: (fun(component: u.renderer.FnComponent, tag: u.renderer.Tag): any),
 ---   unknown?: fun(tag: any): any
 --- }
-function Renderer.tree_match(tree, visitors)-- {{{
+function Renderer.tree_match(tree, visitors) -- {{{
   local function is_tag(x) return type(x) == 'table' and x.kind == 'tag' end
   local function is_tag_arr(x) return type(x) == 'table' and not is_tag(x) end
 
@@ -108,7 +108,7 @@ function Renderer.tree_match(tree, visitors)-- {{{
   else
     return visitors.unknown and visitors.unknown(tree) or error 'unknown value: not a tag'
   end
-end-- }}}
+end -- }}}
 
 --- @private
 --- @param tree u.renderer.Tree
@@ -788,15 +788,15 @@ function Renderer:mount(tree) -- {{{
             or nil
           if old_tree_kind == new_tree_kind then self.component_tree.ctx_by_node[old_tree] = nil end
 
-          --- @param new_tree u.renderer.Tree
           --- @param ctx u.renderer.FnComponentContext
           --- @param rendered_component u.renderer.Tree
-          local function update_ctx_and_recurse(new_tree, ctx, rendered_component)
+          local function update_ctx_and_recurse(ctx, rendered_component)
             self.component_tree.ctx_by_node[new_tree] = ctx
             -- The rendered component could have other components in its tree:
             -- recurse and simplify:
-            local new_tree_simplified = H2.visit_tree(ctx.prev_tree, rendered_component)
-            ctx.prev_tree = new_tree_simplified
+            local new_tree_simplified =
+              M.h('text', ctx.props, H2.visit_tree(ctx.prev_tree, rendered_component))
+            ctx.prev_tree = rendered_component
             return new_tree_simplified
           end
 
@@ -804,13 +804,13 @@ function Renderer:mount(tree) -- {{{
             old_ctx.phase = 'update'
             old_ctx.props = new_tag.attributes
             old_ctx.children = new_tag.children
-            return update_ctx_and_recurse(new_tree, old_ctx, NewC(old_ctx))
+            return update_ctx_and_recurse(old_ctx, NewC(old_ctx))
           else
             local state = tracker.create_signal(nil)
 
             --- @type u.renderer.FnComponentContext
             local ctx = {
-              phase = old_tree and 'update' or 'mount',
+              phase = 'mount',
               props = new_tag.attributes,
               children = new_tag.children,
               state = state --[[@as any]],
@@ -822,7 +822,7 @@ function Renderer:mount(tree) -- {{{
             -- first-renders to initialize the state without firing a
             -- re-render:
             ctx.unsubscribe = state:subscribe(render)
-            return update_ctx_and_recurse(new_tree, ctx, rendered)
+            return update_ctx_and_recurse(ctx, rendered)
           end
         end,
       })
@@ -836,29 +836,33 @@ function Renderer:mount(tree) -- {{{
     --- @param new_arr u.renderer.Node[]
     --- @return u.renderer.Node[]
     function H2.visit_array(old_arr, new_arr)
-      --- @type u.renderer.Node[]
-      local old_to_unmount = {}
       --- @type table<any, u.renderer.Node>
       local old_by_key = {}
-      for _, old in ipairs(old_arr or {}) do
-        local key = Renderer.tree_match(old, {
-          tag = function(t) return t.attributes.key end,
-          component = function(_, t) return t.attributes.key end,
+
+      --- @param tree u.renderer.Tree
+      --- @param idx integer
+      local function get_or_set_key(tree, idx)
+        return Renderer.tree_match(tree, {
+          tag = function(t)
+            if not t.attributes.key then t.attributes.key = idx end
+            return t.attributes.key
+          end,
+          component = function(_, t)
+            if not t.attributes.key then t.attributes.key = idx end
+            return t.attributes.key
+          end,
         })
-        if not key then
-          table.insert(old_to_unmount, old)
-        else
-          old_by_key[key] = old
-        end
+      end
+
+      for idx, old in ipairs(old_arr or {}) do
+        local key = get_or_set_key(old, idx)
+        if key then old_by_key[key] = old end
       end
 
       --- @type u.renderer.Node[]
       local resulting_nodes = {}
-      for _, new in ipairs(new_arr) do
-        local key = Renderer.tree_match(new, {
-          tag = function(t) return t.attributes.key end,
-          component = function(_, t) return t.attributes.key end,
-        })
+      for idx, new in ipairs(new_arr) do
+        local key = get_or_set_key(new, idx)
         local old = nil
         if key then
           old = old_by_key[key]
@@ -867,10 +871,8 @@ function Renderer:mount(tree) -- {{{
         table.insert(resulting_nodes, H2.visit_tree(old, new))
       end
 
+      -- Unmount whatever is left in the "old" map:
       for _, n in pairs(old_by_key) do
-        table.insert(old_to_unmount, n)
-      end
-      for _, n in pairs(old_to_unmount) do
         H2.unmount(n)
       end