diff --git a/README.md b/README.md
index 017c17f..8498111 100644
--- a/README.md
+++ b/README.md
@@ -113,6 +113,7 @@ Built-in provider names:
 - `:PeekstackRestoreAllPopups` — restore all closed popups
 - `:PeekstackCloseAll` — close all popups in the current stack
 - `:PeekstackToggle` — temporarily hide/show all popups in the current stack
+- `:PeekstackZoom` — toggle zoom (maximize the top popup to fill the editor)
 - `:PeekstackHistory` — show popup history and select to restore
 - `:PeekstackQuickPeek [provider]` — quick peek without stacking (default: `lsp.definition`, accepts any registered provider)
 
@@ -127,6 +128,7 @@ Defaults inside popup windows:
 - `<C-v>` — promote to vertical split
 - `<C-t>` — promote to new tab
 - `<leader>os` — open stack view
+- `<C-z>` — toggle zoom (maximize top popup)
 
 Defaults in stack view:
 
@@ -140,6 +142,7 @@ Defaults in stack view:
 - `/` — filter
 - `gg/G` — jump to first/last stack item
 - `j/k` — move cursor by stack item (skip header/preview lines)
+- `z` — toggle zoom (maximize top popup)
 - `?` — help
 - `q` — close
 
@@ -228,6 +231,7 @@ Configure via `require("peekstack").setup({ ... })`.
       promote_vsplit = "<C-v>",
       promote_tab = "<C-t>",
       toggle_stack_view = "<leader>os",
+      zoom = "<C-z>",
     },
   },
   picker = {
diff --git a/doc/peekstack.txt b/doc/peekstack.txt
index b931757..8fe0c06 100644
--- a/doc/peekstack.txt
+++ b/doc/peekstack.txt
@@ -98,6 +98,7 @@ With options:
         promote_vsplit = "<C-v>",
         promote_tab = "<C-t>",
         toggle_stack_view = "<leader>os",
+        zoom = "<C-z>",
       },
     },
     picker = {
@@ -297,6 +298,13 @@ Commands are registered after `setup()` is called.
     again recreates the windows in their original layout. Pushing a new
     popup while hidden automatically restores visibility.
 
+:PeekstackZoom                                      *:PeekstackZoom*
+    Toggle zoom on the top popup. When zoomed, the popup fills the entire
+    editor. Call again to restore the normal layout. The border highlight
+    changes to `PeekstackPopupBorderZoomed` while zoomed. Zoom is
+    automatically cleared when the zoomed popup is closed, a new popup
+    is pushed, or the stack is hidden via |:PeekstackToggle|.
+
 :PeekstackHistory                             *:PeekstackHistory*
     Show popup history and select an entry to restore.
 
@@ -496,6 +504,7 @@ Default keymaps inside popup windows:
     `<C-v>`       Promote to vertical split
     `<C-t>`       Promote to new tab
     `<leader>os`  Toggle the stack view panel
+    `<C-z>`       Toggle zoom (maximize top popup)
 
 Stack view keymaps:
 
@@ -509,6 +518,7 @@ Stack view keymaps:
     `/`           Filter the list
     `gg/G`        Jump to first/last stack item
     `j/k`         Move cursor by stack item (skip header/preview lines)
+    `z`           Toggle zoom (maximize top popup)
     `?`           Toggle help
     `q`           Close the stack view
 
@@ -534,6 +544,7 @@ Title highlights:
 Popup border highlights:
     PeekstackPopupBorder           Unfocused popup border (links to FloatBorder)
     PeekstackPopupBorderFocused    Focused popup border (links to Function)
+    PeekstackPopupBorderZoomed     Zoomed popup border (links to WarningMsg)
 
 Stack view highlights:
     PeekstackStackViewIndex        Entry index number (links to LineNr)
diff --git a/lua/peekstack/commands.lua b/lua/peekstack/commands.lua
index c9b1f5c..3c1a037 100644
--- a/lua/peekstack/commands.lua
+++ b/lua/peekstack/commands.lua
@@ -14,6 +14,7 @@ local COMMAND_NAMES = {
   "PeekstackCloseAll",
   "PeekstackQuickPeek",
   "PeekstackToggle",
+  "PeekstackZoom",
 }
 
 ---@param session PeekstackSession|table
@@ -181,6 +182,13 @@ function M.setup()
     end
   end, {})
 
+  vim.api.nvim_create_user_command("PeekstackZoom", function()
+    local toggled = require("peekstack.core.stack").toggle_zoom()
+    if not toggled then
+      notify.info("No popups in the current stack")
+    end
+  end, {})
+
   vim.api.nvim_create_user_command("PeekstackQuickPeek", function(opts)
     local provider = opts.args and opts.args ~= "" and opts.args or "lsp.definition"
     require("peekstack").peek(provider, { mode = "quick" })
diff --git a/lua/peekstack/config.lua b/lua/peekstack/config.lua
index 6679e62..0f1c716 100644
--- a/lua/peekstack/config.lua
+++ b/lua/peekstack/config.lua
@@ -156,6 +156,7 @@ M.defaults = {
       promote_vsplit = "<C-v>",
       promote_tab = "<C-t>",
       toggle_stack_view = "<leader>os",
+      zoom = "<C-z>",
     },
   },
   picker = {
diff --git a/lua/peekstack/core/events.lua b/lua/peekstack/core/events.lua
index dd92e40..b6c3658 100644
--- a/lua/peekstack/core/events.lua
+++ b/lua/peekstack/core/events.lua
@@ -125,12 +125,11 @@ function M.setup()
   local close_events = cfg.ui.quick_peek and cfg.ui.quick_peek.close_events
     or { "CursorMoved", "InsertEnter", "BufLeave", "WinLeave" }
 
-  for _, event in ipairs(close_events) do
-    vim.api.nvim_create_autocmd(event, {
-      group = group,
-      callback = close_ephemeral_popups,
-    })
-  end
+  -- Merge all close events into a single autocmd for efficiency
+  vim.api.nvim_create_autocmd(close_events, {
+    group = group,
+    callback = close_ephemeral_popups,
+  })
 
   vim.api.nvim_create_autocmd("User", {
     group = group,
diff --git a/lua/peekstack/core/layout.lua b/lua/peekstack/core/layout.lua
index 173756a..4e943bf 100644
--- a/lua/peekstack/core/layout.lua
+++ b/lua/peekstack/core/layout.lua
@@ -67,6 +67,22 @@ function M.compute(index)
   }
 end
 
+---Compute fullscreen layout for zoomed popup.
+---@param popup_count integer  number of popups in the stack
+---@return PeekstackLayoutResult
+function M.compute_zoom(popup_count)
+  local columns = vim.o.columns
+  local lines = vim.o.lines - vim.o.cmdheight
+  local base = config.get().ui.layout.zindex_base
+  return {
+    width = columns,
+    height = lines,
+    row = 0,
+    col = 0,
+    zindex = base + popup_count + 1,
+  }
+end
+
 ---@param winid integer
 ---@param is_focused boolean
 local function set_popup_winhighlight(winid, is_focused)
@@ -89,28 +105,52 @@ local function focused_popup_winid(stack)
   return nil
 end
 
+---@param winid integer
+---@param is_zoomed boolean
+local function set_popup_zoom_winhighlight(winid, is_zoomed)
+  if not vim.api.nvim_win_is_valid(winid) then
+    return
+  end
+  if is_zoomed then
+    vim.wo[winid].winhighlight = "FloatBorder:PeekstackPopupBorderZoomed"
+  end
+end
+
 ---@param stack PeekstackStackModel
 function M.reflow(stack)
   local focused_winid = focused_popup_winid(stack)
   local base = config.get().ui.layout.zindex_base
   local top = base + #stack.popups
+  local zoomed_id = stack.zoomed_id
   for idx, popup in ipairs(stack.popups) do
     if popup.winid and vim.api.nvim_win_is_valid(popup.winid) then
       local is_focused = focused_winid ~= nil and popup.winid == focused_winid
-      local layout = M.compute(idx)
-      local z = layout.zindex
-      if is_focused then
+      local is_zoomed = zoomed_id ~= nil and popup.id == zoomed_id
+
+      local lo
+      if is_zoomed then
+        lo = M.compute_zoom(#stack.popups)
+      else
+        lo = M.compute(idx)
+      end
+
+      local z = lo.zindex
+      if not is_zoomed and is_focused then
         z = top
       end
       local win_opts = vim.tbl_extend("force", popup.win_opts or {}, {
-        row = layout.row,
-        col = layout.col,
-        width = layout.width,
-        height = layout.height,
+        row = lo.row,
+        col = lo.col,
+        width = lo.width,
+        height = lo.height,
         zindex = z,
       })
       vim.api.nvim_win_set_config(popup.winid, win_opts)
-      set_popup_winhighlight(popup.winid, is_focused)
+      if is_zoomed then
+        set_popup_zoom_winhighlight(popup.winid, true)
+      else
+        set_popup_winhighlight(popup.winid, is_focused)
+      end
     end
   end
 end
@@ -126,12 +166,24 @@ function M.update_focus_zindex(stack, focused_winid)
   local ui = config.get().ui
   local base = ui.layout.zindex_base
   local top = base + #stack.popups
+  local zoomed_id = stack.zoomed_id
 
   for idx, popup in ipairs(stack.popups) do
     if popup.winid and vim.api.nvim_win_is_valid(popup.winid) then
       local is_focused = popup.winid == focused_winid
-      local z = is_focused and top or (base + idx - 1)
-      local lo = M.compute(idx)
+      local is_zoomed = zoomed_id ~= nil and popup.id == zoomed_id
+
+      local lo
+      if is_zoomed then
+        lo = M.compute_zoom(#stack.popups)
+      else
+        lo = M.compute(idx)
+      end
+
+      local z = lo.zindex
+      if not is_zoomed and is_focused then
+        z = top
+      end
       local win_opts = vim.tbl_extend("force", popup.win_opts or {}, {
         row = lo.row,
         col = lo.col,
@@ -140,7 +192,11 @@ function M.update_focus_zindex(stack, focused_winid)
         zindex = z,
       })
       pcall(vim.api.nvim_win_set_config, popup.winid, win_opts)
-      set_popup_winhighlight(popup.winid, is_focused)
+      if is_zoomed then
+        set_popup_zoom_winhighlight(popup.winid, true)
+      else
+        set_popup_winhighlight(popup.winid, is_focused)
+      end
     end
   end
 end
diff --git a/lua/peekstack/core/stack.lua b/lua/peekstack/core/stack.lua
index dbcf4cc..f2002ef 100644
--- a/lua/peekstack/core/stack.lua
+++ b/lua/peekstack/core/stack.lua
@@ -283,6 +283,10 @@ function M.push(location, opts)
   if stack.hidden then
     M.toggle(stack.root_winid)
   end
+  -- Clear zoom before pushing so the new top popup gets normal layout
+  if stack.zoomed_id then
+    stack.zoomed_id = nil
+  end
   local model = popup.create(location, create_opts)
   if not model then
     return nil
@@ -325,6 +329,10 @@ end
 local function close_stack_item(stack, idx, item)
   local current_win = vim.api.nvim_get_current_win()
   local should_restore_focus = item.winid == current_win and vim.w[current_win].peekstack_popup_id ~= nil
+  -- Clear zoom if the zoomed popup is being closed
+  if stack.zoomed_id == item.id then
+    stack.zoomed_id = nil
+  end
   -- Remove from popups BEFORE closing the window to prevent
   -- WinClosed autocmd from re-entering and processing the same popup.
   table.remove(stack.popups, idx)
@@ -650,6 +658,9 @@ function M.handle_win_closed(winid)
           if stack.focused_id == item.id then
             focused_removed = true
           end
+          if stack.zoomed_id == item.id then
+            stack.zoomed_id = nil
+          end
           emit_popup_event("PeekstackClose", item, root_winid)
           feedback.highlight_origin(item.origin)
           table.remove(stack.popups, idx)
@@ -719,6 +730,9 @@ function M.handle_buf_wipeout(bufnr)
     for idx = #stack.popups, 1, -1 do
       local item = stack.popups[idx]
       if item.bufnr == bufnr then
+        if stack.zoomed_id == item.id then
+          stack.zoomed_id = nil
+        end
         unindex_popup(item)
         table.remove(stack.popups, idx)
       end
@@ -795,6 +809,9 @@ function M.handle_origin_wipeout(bufnr)
     for idx = #stack.popups, 1, -1 do
       local item = stack.popups[idx]
       if should_close_for_origin(item) then
+        if stack.zoomed_id == item.id then
+          stack.zoomed_id = nil
+        end
         popup.close(item)
         unindex_popup(item)
         table.remove(stack.popups, idx)
@@ -855,6 +872,8 @@ function M.toggle(winid)
   end
 
   if not stack.hidden then
+    -- Clear zoom state before hiding
+    stack.zoomed_id = nil
     -- Move focus back to root window before hiding
     if vim.api.nvim_win_is_valid(stack.root_winid) then
       vim.api.nvim_set_current_win(stack.root_winid)
@@ -897,11 +916,41 @@ function M.is_hidden(winid)
   return stack.hidden == true
 end
 
+---Toggle zoom on the top popup.  When zoomed, the popup fills the
+---entire editor.  Calling again restores the normal layout.
+---@param winid? integer
+---@return boolean
+function M.toggle_zoom(winid)
+  deps()
+  local stack = ensure_stack(winid)
+  if #stack.popups == 0 or stack.hidden then
+    return false
+  end
+
+  local top = stack.popups[#stack.popups]
+  if stack.zoomed_id == top.id then
+    stack.zoomed_id = nil
+  else
+    stack.zoomed_id = top.id
+  end
+  layout.reflow(stack)
+  return true
+end
+
+---Check whether the current stack has a zoomed popup.
+---@param winid? integer
+---@return boolean
+function M.is_zoomed(winid)
+  local stack = ensure_stack(winid)
+  return stack.zoomed_id ~= nil
+end
+
 --- Close all popups in the current (or given) window's stack.
 ---@param winid? integer
 function M.close_all(winid)
   deps()
   local stack = ensure_stack(winid)
+  stack.zoomed_id = nil
   -- When hidden, windows are already closed; just clear hidden state
   -- and record history for each popup.
   if stack.hidden then
diff --git a/lua/peekstack/init.lua b/lua/peekstack/init.lua
index 04857c2..77cfb0f 100644
--- a/lua/peekstack/init.lua
+++ b/lua/peekstack/init.lua
@@ -32,6 +32,7 @@ local function set_hl()
   vim.api.nvim_set_hl(0, "PeekstackStackViewLine", { default = true, link = "LineNr" })
   vim.api.nvim_set_hl(0, "PeekstackPopupBorder", { default = true, link = "FloatBorder" })
   vim.api.nvim_set_hl(0, "PeekstackPopupBorderFocused", { default = true, link = "Function" })
+  vim.api.nvim_set_hl(0, "PeekstackPopupBorderZoomed", { default = true, link = "WarningMsg" })
   vim.api.nvim_set_hl(0, "PeekstackTitleKindError", { default = true, link = "DiagnosticError" })
   vim.api.nvim_set_hl(0, "PeekstackTitleKindWarn", { default = true, link = "DiagnosticWarn" })
   vim.api.nvim_set_hl(0, "PeekstackTitleKindInfo", { default = true, link = "DiagnosticInfo" })
diff --git a/lua/peekstack/types.lua b/lua/peekstack/types.lua
index b2cd982..a352c7f 100644
--- a/lua/peekstack/types.lua
+++ b/lua/peekstack/types.lua
@@ -63,6 +63,7 @@
 ---@field layout_state any
 ---@field focused_id integer?
 ---@field hidden boolean?
+---@field zoomed_id integer?
 
 ---@class PeekstackUserEventData
 ---@field event string
@@ -250,6 +251,7 @@
 ---@field promote_vsplit string
 ---@field promote_tab string
 ---@field toggle_stack_view string
+---@field zoom string
 
 ---@class PeekstackConfigUI
 ---@field layout PeekstackConfigLayout
diff --git a/lua/peekstack/ui/diagnostics.lua b/lua/peekstack/ui/diagnostics.lua
index ac06baf..dd03384 100644
--- a/lua/peekstack/ui/diagnostics.lua
+++ b/lua/peekstack/ui/diagnostics.lua
@@ -8,39 +8,16 @@ local NS = vim.api.nvim_create_namespace("peekstack_diagnostics")
 ---@field ids integer[]
 
 ---@param kind? integer
+---@param prefix string "DiagnosticVirtualText" or "DiagnosticUnderline"
 ---@return string
-local function virtual_text_hl(kind)
-  if kind == vim.diagnostic.severity.ERROR then
-    return "DiagnosticVirtualTextError"
-  end
-  if kind == vim.diagnostic.severity.WARN then
-    return "DiagnosticVirtualTextWarn"
-  end
-  if kind == vim.diagnostic.severity.INFO then
-    return "DiagnosticVirtualTextInfo"
-  end
-  if kind == vim.diagnostic.severity.HINT then
-    return "DiagnosticVirtualTextHint"
-  end
-  return "DiagnosticVirtualTextInfo"
-end
-
----@param kind? integer
----@return string
-local function underline_hl(kind)
-  if kind == vim.diagnostic.severity.ERROR then
-    return "DiagnosticUnderlineError"
-  end
-  if kind == vim.diagnostic.severity.WARN then
-    return "DiagnosticUnderlineWarn"
-  end
-  if kind == vim.diagnostic.severity.INFO then
-    return "DiagnosticUnderlineInfo"
-  end
-  if kind == vim.diagnostic.severity.HINT then
-    return "DiagnosticUnderlineHint"
-  end
-  return "DiagnosticUnderlineInfo"
+local function severity_hl(kind, prefix)
+  local suffix = ({
+    [vim.diagnostic.severity.ERROR] = "Error",
+    [vim.diagnostic.severity.WARN] = "Warn",
+    [vim.diagnostic.severity.INFO] = "Info",
+    [vim.diagnostic.severity.HINT] = "Hint",
+  })[kind] or "Info"
+  return prefix .. suffix
 end
 
 ---@param text string
@@ -103,7 +80,7 @@ function M.decorate(popup)
   local ids = {}
 
   local virt_lines = {}
-  local virt_hl = virtual_text_hl(location.kind)
+  local virt_hl = severity_hl(location.kind, "DiagnosticVirtualText")
   for _, msg in ipairs(split_message(text)) do
     local msg_text = msg ~= "" and msg or " "
     table.insert(virt_lines, { { msg_text, virt_hl } })
@@ -117,7 +94,7 @@ function M.decorate(popup)
     table.insert(ids, id)
   end
 
-  local underline = underline_hl(location.kind)
+  local underline = severity_hl(location.kind, "DiagnosticUnderline")
   if underline ~= "" then
     local id = vim.api.nvim_buf_set_extmark(bufnr, NS, line, col, {
       end_row = end_line,
diff --git a/lua/peekstack/ui/keymaps.lua b/lua/peekstack/ui/keymaps.lua
index dc1f584..2ca0eab 100644
--- a/lua/peekstack/ui/keymaps.lua
+++ b/lua/peekstack/ui/keymaps.lua
@@ -89,6 +89,11 @@ function M.apply_popup(popup)
   map(popup.bufnr, keys.toggle_stack_view, function()
     stack_view.toggle()
   end, "Peekstack stack view")
+
+  map(popup.bufnr, keys.zoom, function()
+    local stack = require("peekstack.core.stack")
+    stack.toggle_zoom()
+  end, "Peekstack zoom")
 end
 
 return M
diff --git a/lua/peekstack/ui/stack_view/keymaps.lua b/lua/peekstack/ui/stack_view/keymaps.lua
index e69f8f2..d9d9422 100644
--- a/lua/peekstack/ui/stack_view/keymaps.lua
+++ b/lua/peekstack/ui/stack_view/keymaps.lua
@@ -187,6 +187,7 @@ local function toggle_help(s, deps)
     "/     Filter list",
     "gg/G  Jump to first/last stack item",
     "j/k   Move cursor by stack item",
+    "z     Toggle zoom (maximize top popup)",
     "q     Close stack view",
     "?     Toggle this help",
   }
@@ -460,6 +461,11 @@ function M.apply(s, deps)
     end)
   end, { buffer = s.bufnr, nowait = true, silent = true })
 
+  vim.keymap.set("n", "z", function()
+    local stack = require("peekstack.core.stack")
+    stack.toggle_zoom(s.root_winid)
+  end, { buffer = s.bufnr, nowait = true, silent = true })
+
   vim.keymap.set("n", "?", function()
     toggle_help(s, deps)
   end, { buffer = s.bufnr, nowait = true, silent = true })
diff --git a/lua/peekstack/util/fs.lua b/lua/peekstack/util/fs.lua
index 5f11faf..db8a99c 100644
--- a/lua/peekstack/util/fs.lua
+++ b/lua/peekstack/util/fs.lua
@@ -2,6 +2,20 @@ local M = {}
 
 ---@type string?
 local cached_repo_root = nil
+
+---Check if a window ID is valid (non-nil and exists)
+---@param winid integer?
+---@return boolean
+function M.win_is_valid(winid)
+  return winid ~= nil and vim.api.nvim_win_is_valid(winid)
+end
+
+---Check if a buffer number is valid (non-nil and exists)
+---@param bufnr integer?
+---@return boolean
+function M.buf_is_valid(bufnr)
+  return bufnr ~= nil and vim.api.nvim_buf_is_valid(bufnr)
+end
 ---@type string?
 local cached_repo_cwd = nil
 ---@type string?
@@ -84,9 +98,7 @@ end
 ---@param path string
 ---@return string
 function M.ensure_dir(path)
-  if vim.fn.isdirectory(path) == 1 then
-    return path
-  end
+  -- mkdir with "p" flag is atomic and handles existing directories
   vim.fn.mkdir(path, "p")
   return path
 end
diff --git a/tests/fs_spec.lua b/tests/fs_spec.lua
index 6346ac2..79a6986 100644
--- a/tests/fs_spec.lua
+++ b/tests/fs_spec.lua
@@ -30,6 +30,54 @@ describe("fs", function()
     end)
   end)
 
+  describe("win_is_valid", function()
+    it("returns false for nil input", function()
+      assert.is_false(fs.win_is_valid(nil))
+    end)
+
+    it("returns true for current window", function()
+      assert.is_true(fs.win_is_valid(vim.api.nvim_get_current_win()))
+    end)
+
+    it("returns false for closed window", function()
+      local bufnr = vim.api.nvim_create_buf(false, true)
+      local winid = vim.api.nvim_open_win(bufnr, false, {
+        relative = "editor",
+        row = 0,
+        col = 0,
+        width = 1,
+        height = 1,
+        style = "minimal",
+      })
+
+      assert.is_true(fs.win_is_valid(winid))
+      vim.api.nvim_win_close(winid, true)
+      assert.is_false(fs.win_is_valid(winid))
+
+      if vim.api.nvim_buf_is_valid(bufnr) then
+        vim.api.nvim_buf_delete(bufnr, { force = true })
+      end
+    end)
+  end)
+
+  describe("buf_is_valid", function()
+    it("returns false for nil input", function()
+      assert.is_false(fs.buf_is_valid(nil))
+    end)
+
+    it("returns true for current buffer", function()
+      assert.is_true(fs.buf_is_valid(vim.api.nvim_get_current_buf()))
+    end)
+
+    it("returns false for deleted buffer", function()
+      local bufnr = vim.api.nvim_create_buf(false, true)
+      assert.is_true(fs.buf_is_valid(bufnr))
+
+      vim.api.nvim_buf_delete(bufnr, { force = true })
+      assert.is_false(fs.buf_is_valid(bufnr))
+    end)
+  end)
+
   describe("scope_path", function()
     local original_ensure_dir
 
diff --git a/tests/zoom_spec.lua b/tests/zoom_spec.lua
new file mode 100644
index 0000000..105ae60
--- /dev/null
+++ b/tests/zoom_spec.lua
@@ -0,0 +1,177 @@
+local stack = require("peekstack.core.stack")
+local config = require("peekstack.config")
+local events = require("peekstack.core.events")
+local helpers = require("tests.helpers")
+
+describe("stack.toggle_zoom", function()
+  before_each(function()
+    stack._reset()
+    config.setup({})
+    events.setup()
+  end)
+
+  after_each(function()
+    local s = stack.current_stack()
+    if s.hidden then
+      stack.toggle()
+    end
+    for i = #s.popups, 1, -1 do
+      stack.close(s.popups[i].id)
+    end
+    stack._reset()
+  end)
+
+  it("returns false on empty stack", function()
+    assert.is_false(stack.toggle_zoom())
+    assert.is_false(stack.is_zoomed())
+  end)
+
+  it("zooms the top popup to fullscreen", function()
+    local loc = helpers.make_location()
+    stack.push(loc)
+    local m2 = stack.push(loc)
+
+    assert.is_true(stack.toggle_zoom())
+    assert.is_true(stack.is_zoomed())
+
+    local cfg = vim.api.nvim_win_get_config(m2.winid)
+    local expected_w = vim.o.columns
+    local expected_h = vim.o.lines - vim.o.cmdheight
+
+    -- The zoomed popup should be at least close to fullscreen.
+    -- nvim_win_set_config may adjust for border, so allow tolerance.
+    assert.is_true(cfg.width >= expected_w - 2)
+    assert.is_true(cfg.height >= expected_h - 2)
+  end)
+
+  it("unzooms on second toggle", function()
+    local loc = helpers.make_location()
+    stack.push(loc)
+    local m2 = stack.push(loc)
+
+    stack.toggle_zoom()
+    assert.is_true(stack.is_zoomed())
+
+    stack.toggle_zoom()
+    assert.is_false(stack.is_zoomed())
+
+    -- After unzoom, size should match normal layout
+    local layout = require("peekstack.core.layout")
+    local expected = layout.compute(2)
+    local cfg = vim.api.nvim_win_get_config(m2.winid)
+    assert.equals(expected.width, cfg.width)
+    assert.equals(expected.height, cfg.height)
+  end)
+
+  it("clears zoom when the zoomed popup is closed", function()
+    local loc = helpers.make_location()
+    stack.push(loc)
+    local m2 = stack.push(loc)
+
+    stack.toggle_zoom()
+    assert.is_true(stack.is_zoomed())
+
+    stack.close(m2.id)
+    assert.is_false(stack.is_zoomed())
+  end)
+
+  it("clears zoom on push", function()
+    local loc = helpers.make_location()
+    stack.push(loc)
+
+    stack.toggle_zoom()
+    assert.is_true(stack.is_zoomed())
+
+    stack.push(loc)
+    assert.is_false(stack.is_zoomed())
+  end)
+
+  it("clears zoom when hiding via toggle visibility", function()
+    local loc = helpers.make_location()
+    stack.push(loc)
+
+    stack.toggle_zoom()
+    assert.is_true(stack.is_zoomed())
+
+    stack.toggle() -- hide
+    assert.is_true(stack.is_hidden())
+    assert.is_false(stack.is_zoomed())
+  end)
+
+  it("returns false when stack is hidden", function()
+    local loc = helpers.make_location()
+    stack.push(loc)
+
+    stack.toggle() -- hide
+    assert.is_false(stack.toggle_zoom())
+    assert.is_false(stack.is_zoomed())
+  end)
+
+  it("clears zoom on close_all", function()
+    local loc = helpers.make_location()
+    stack.push(loc)
+    stack.push(loc)
+
+    stack.toggle_zoom()
+    assert.is_true(stack.is_zoomed())
+
+    stack.close_all()
+    assert.is_false(stack.is_zoomed())
+  end)
+
+  it("sets zoomed border highlight", function()
+    local loc = helpers.make_location()
+    stack.push(loc)
+    local m = stack.push(loc)
+
+    stack.toggle_zoom()
+
+    local whl = vim.wo[m.winid].winhighlight
+    assert.truthy(whl:find("PeekstackPopupBorderZoomed"))
+  end)
+
+  it("restores normal border highlight on unzoom", function()
+    local loc = helpers.make_location()
+    stack.push(loc)
+    local m = stack.push(loc)
+
+    stack.toggle_zoom()
+    stack.toggle_zoom()
+
+    local whl = vim.wo[m.winid].winhighlight
+    assert.falsy(whl:find("PeekstackPopupBorderZoomed"))
+  end)
+
+  it("zoomed zindex is above all popups regardless of zindex_base", function()
+    config.setup({ ui = { layout = { zindex_base = 300 } } })
+    events.setup()
+    local loc = helpers.make_location()
+    stack.push(loc)
+    local m2 = stack.push(loc)
+
+    stack.toggle_zoom()
+
+    local s = stack.current_stack()
+    local zoomed_cfg = vim.api.nvim_win_get_config(m2.winid)
+    -- Zoomed zindex must be higher than any non-zoomed popup
+    for _, p in ipairs(s.popups) do
+      if p.id ~= m2.id and p.winid and vim.api.nvim_win_is_valid(p.winid) then
+        local other_cfg = vim.api.nvim_win_get_config(p.winid)
+        assert.is_true(zoomed_cfg.zindex > other_cfg.zindex)
+      end
+    end
+  end)
+
+  it("clears zoom when popup window is closed externally", function()
+    local loc = helpers.make_location()
+    stack.push(loc)
+    local m2 = stack.push(loc)
+
+    stack.toggle_zoom()
+    assert.is_true(stack.is_zoomed())
+
+    -- Simulate external close (e.g., nvim_win_close)
+    vim.api.nvim_win_close(m2.winid, true)
+    assert.is_false(stack.is_zoomed())
+  end)
+end)