feat: compact dropdown popover style

lua/input-form/inputs/select.lua

@@ -173,17 +173,40 @@ function M:open_dropdown()
   local max_h = config.options.select.max_height
   local height = math.min(#lines, max_h)
 
-  -- Position the dropdown's top border immediately beneath the input's bottom
-  -- border. Content origin row = (input content row) + (input bottom border = 1)
-  -- + (dropdown top border = 1) + 1 = self._layout.row + 3.
+  -- Prefer stitching the dropdown's top border into the select's bottom
+  -- border for a compact, merged look:
+  --
+  --   ╭─ Label ─────╮
+  --   │ Option 1  ⌃ │
+  --   ├─────────────┤   <- shared row (dropdown's top border with T-junction
+  --   │ Option 1    │      connectors, overlaid on the select's bottom)
+  --   │ Option 2    │
+  --   ╰─────────────╯
+  --
+  -- The dropdown is positioned so its top border row coincides with the
+  -- select's bottom border row. With a higher zindex, the dropdown's top
+  -- border (├─┤ / ╠═╣) wins, producing the visible T-junctions.
+  local cfg_border = config.options.window.border
+  local merged_border = utils.merged_top_border(cfg_border)
+  local dropdown_row, dropdown_border
+  if merged_border then
+    dropdown_row = self._layout.row + 2
+    dropdown_border = merged_border
+  else
+    -- Fallback for unmergeable borders (`"none"`, `"shadow"`, ...): keep the
+    -- dropdown on its own, one row below the select.
+    dropdown_row = self._layout.row + 3
+    dropdown_border = cfg_border
+  end
+
   self.dropdown_win = vim.api.nvim_open_win(self.dropdown_buf, true, {
     relative = "editor",
-    row = self._layout.row + 3,
+    row = dropdown_row,
     col = self._layout.col,
     width = self._layout.width,
     height = height,
     style = "minimal",
-    border = "rounded",
+    border = dropdown_border,
     focusable = true,
     zindex = 100,
   })

lua/input-form/utils.lua

@@ -43,6 +43,57 @@ end
 --- their UI plugins' exclusion lists as a fallback.
 M.FORM_FILETYPE = "input-form"
 
+--- Character sets for the built-in border styles accepted by `nvim_open_win`.
+--- Order is clockwise from top-left: TL, T, TR, R, BR, B, BL, L.
+local BORDER_CHARS = {
+  rounded = { "╭", "─", "╮", "│", "╯", "─", "╰", "│" },
+  single = { "┌", "─", "┐", "│", "┘", "─", "└", "│" },
+  double = { "╔", "═", "╗", "║", "╝", "═", "╚", "║" },
+  solid = { " ", " ", " ", " ", " ", " ", " ", " " },
+}
+
+-- T-junction connectors used to replace the top corners when stitching two
+-- boxes together (the bottom of box A and the top of box B share a row).
+local MERGE_CONNECTORS = {
+  rounded = { left = "├", right = "┤" },
+  single = { left = "├", right = "┤" },
+  double = { left = "╠", right = "╣" },
+  solid = { left = " ", right = " " },
+}
+
+--- Build an 8-element border array whose top row is a T-junction stitching
+--- into the bottom of a parent box above it. Accepts either one of the
+--- built-in border style names or an existing 8-element border array.
+---
+--- Returns `nil` for unrecognised / non-mergeable borders (e.g. `"none"`,
+--- `"shadow"`) so the caller can fall back to an unmerged layout.
+---@param border string|table
+---@return table|nil
+function M.merged_top_border(border)
+  local chars, connectors
+  if type(border) == "string" then
+    chars = BORDER_CHARS[border]
+    connectors = MERGE_CONNECTORS[border]
+  elseif type(border) == "table" and #border == 8 then
+    chars = vim.deepcopy(border)
+    -- Best-effort fallback for custom arrays: use the straight T's.
+    connectors = { left = "├", right = "┤" }
+  end
+  if not chars or not connectors then
+    return nil
+  end
+  return {
+    connectors.left,
+    chars[2],
+    connectors.right,
+    chars[4],
+    chars[5],
+    chars[6],
+    chars[7],
+    chars[8],
+  }
+end
+
 local _excluded_registered = false
 
 -- Append `ft` to a list-shaped config field if missing.

tests/test_inputs_select.lua

@@ -113,6 +113,27 @@ T["select input"]["uses custom chevrons from config"] = function()
   helpers.expect.no_match(open_line, "⌃")
 end
 
+T["select input"]["dropdown border merges into select's bottom border"] = function()
+  -- `rounded` default → T-junctions are ├ and ┤.
+  child.lua([[
+    _G.t = _G.mk('a')
+    _G.t:mount({ row = 5, col = 5, width = 30 })
+    _G.t._layout = { row = 10, col = 5, width = 30 }
+    _G.t:open_dropdown()
+  ]])
+  local cfg = child.lua_get([[vim.api.nvim_win_get_config(_G.t.dropdown_win)]])
+  -- Dropdown row must overlap the select's bottom border row (= layout.row + 2
+  -- for the content origin, putting the top border at layout.row + 1).
+  eq(cfg.row, 12)
+  -- Border is an 8-element array with T-junctions in the top corners.
+  eq(type(cfg.border), "table")
+  -- nvim_win_get_config returns borders as { { char, hl_group }, ... }.
+  local tl = type(cfg.border[1]) == "table" and cfg.border[1][1] or cfg.border[1]
+  local tr = type(cfg.border[3]) == "table" and cfg.border[3][1] or cfg.border[3]
+  eq(tl, "├")
+  eq(tr, "┤")
+end
+
 T["select input"]["rejects empty options list"] = function()
   local ok = child.lua_get([[
     (function()