Apply facing-line rule to input border as well

Visible input border suppressed separator even when it draws no line
toward list section (bottom in default layout, top in reverse, vertical).
Suppress only when input border has a line on the list side; otherwise
fall through to adjacent section check.

- Requires header window to have content before treating it as facing
  section; hasHeaderWindow returns true with empty header when input
  border is visible
- Extract inputBorderFacesList and resolveInputBorderShape shared by
  options and terminal
- Add tests for separator suppression rules

test/test_layout.rb

@@ -1228,6 +1228,115 @@ class TestLayout < TestInteractive
     end
   end
 
+  def test_separator_with_input_border
+    # Border line below input does not face the list; separator is shown
+    tmux.send_keys %(seq 100 | #{FZF} --input-border bottom), :Enter
+    block = <<~BLOCK
+      > 1
+        100/100 ──
+      >
+      ────────────
+    BLOCK
+    tmux.until { assert_block(block, it) }
+    teardown
+    setup
+
+    # List border faces the input instead; separator is hidden
+    tmux.send_keys %(seq 100 | #{FZF} --input-border bottom --list-border rounded), :Enter
+    block = <<~BLOCK
+      │ > 1
+      ╰────────────
+          100/100
+        >
+      ─────────────
+    BLOCK
+    tmux.until { assert_block(block, it) }
+    teardown
+    setup
+
+    # Border line above input faces the list; separator is hidden
+    tmux.send_keys %(seq 100 | #{FZF} --input-border top), :Enter
+    block = <<~BLOCK
+      > 1
+      ────────────
+        100/100
+      >
+    BLOCK
+    tmux.until { assert_block(block, it) }
+  end
+
+  def test_separator_with_input_border_reverse
+    # Border line above input does not face the list; separator is shown
+    tmux.send_keys %(seq 100 | #{FZF} --layout reverse --input-border top), :Enter
+    block = <<~BLOCK
+      ────────────
+      >
+        100/100 ──
+      > 1
+    BLOCK
+    tmux.until { assert_block(block, it) }
+    teardown
+    setup
+
+    # Border line below input faces the list; separator is hidden
+    tmux.send_keys %(seq 100 | #{FZF} --layout reverse --input-border bottom), :Enter
+    block = <<~BLOCK
+      >
+        100/100
+      ────────────
+      > 1
+    BLOCK
+    tmux.until { assert_block(block, it) }
+  end
+
+  def test_separator_with_preview_next
+    # Preview window at 'next' position sits next to the input section, so the
+    # header border does not hide the separator
+    tmux.send_keys %(seq 100 | #{FZF} --header foo --header-border rounded --no-list-border --preview 'echo hi' --preview-window next,1,border-none), :Enter
+    block = <<~BLOCK
+      ╰────────────
+      hi
+        100/100 ───
+      >
+    BLOCK
+    tmux.until { assert_block(block, it) }
+    teardown
+    setup
+
+    # Same when 'next' position comes from a threshold alternative
+    tmux.send_keys %(seq 100 | #{FZF} --header foo --header-border rounded --no-list-border --preview 'echo hi' --preview-window 'up,40%,<9999(next,1,border-none)'), :Enter
+    tmux.until { assert_block(block, it) }
+  end
+
+  def test_separator_with_inline_header_border
+    # Inline header is drawn inside the list border; bare header lines sit
+    # next to the input section, so the separator is shown
+    tmux.send_keys %(seq 100 | #{FZF} --list-border rounded --header foo --header-lines 2 --header-border inline --header-lines-border none), :Enter
+    block = <<~BLOCK
+      ╰────────────
+          2
+          1
+          98/98 ───
+        >
+    BLOCK
+    tmux.until { assert_block(block, it) }
+    teardown
+    setup
+
+    # Header lines are embedded in the list border along with the inline
+    # header; the list border faces the input section and hides the separator
+    tmux.send_keys %(seq 100 | #{FZF} --list-border rounded --header foo --header-lines 2 --header-border inline), :Enter
+    block = <<~BLOCK
+      │   2
+      │   1
+      │   foo
+      ╰────────────
+          98/98
+        >
+    BLOCK
+    tmux.until { assert_block(block, it) }
+  end
+
   def test_header_border_no_pointer_and_marker
     tmux.send_keys %(seq 10 | #{FZF} --header-lines 1 --header-border sharp --no-list-border --pointer '' --marker ''), :Enter
     block = <<~BLOCK