[meld] dirdiff: Make new/deleted file comparison open in two-pane mode (#492)

[Kai Willadsen <kaiw src gnome org>]

commit ab0de11dc5d754edb97d925d99c2a1d23ce352c4
Author: Kai Willadsen <kai willadsen gmail com>
Date:   Sun Jan 9 11:18:05 2022 +1000

    dirdiff: Make new/deleted file comparison open in two-pane mode (#492)
    
    The issue being addressed here is that once the comparison has been
    opened, there's no clear indication of whether it was the left or the
    right pane that was missing from the folder comparison.
    
    The down-side to making us always open in two-pane mode is that we lose
    the screen real estate of the single-pane view, but the benefits in
    consistency feel like they outweigh that consideration. If a user really
    wants a full-size view of an added/removed file without the screen
    split, the Open Externally action is already available.

 meld/dirdiff.py        |  9 +++++----
 meld/iohelpers.py      |  4 ++--
 test/test_iohelpers.py | 13 ++++++++++---
 3 files changed, 17 insertions(+), 9 deletions(-)
---
diff --git a/meld/dirdiff.py b/meld/dirdiff.py
index 9bf7a11e..1a6b7f09 100644
--- a/meld/dirdiff.py
+++ b/meld/dirdiff.py
@@ -1376,10 +1376,11 @@ class DirDiff(Gtk.VBox, tree.TreeviewCommon, MeldDoc):
         if not rows[pane]:
             return
         if os.path.isfile(rows[pane]):
-            self.create_diff_signal.emit(
-                [Gio.File.new_for_path(r) for r in rows if os.path.isfile(r)],
-                {}
-            )
+            diff_gfiles = [
+                Gio.File.new_for_path(r) if os.path.isfile(r) else None
+                for r in rows
+            ]
+            self.create_diff_signal.emit(diff_gfiles, {})
         elif os.path.isdir(rows[pane]):
             if view.row_expanded(path):
                 view.collapse_row(path)
diff --git a/meld/iohelpers.py b/meld/iohelpers.py
index 1a824128..a7882386 100644
--- a/meld/iohelpers.py
+++ b/meld/iohelpers.py
@@ -122,10 +122,10 @@ def prompt_save_filename(
 
 
 def find_shared_parent_path(
-    paths: Sequence[Gio.File],
+    paths: Sequence[Optional[Gio.File]],
 ) -> Optional[Gio.File]:
 
-    if not paths or not paths[0]:
+    if not paths or not paths[0] or any(path is None for path in paths):
         return None
 
     current_parent = paths[0].get_parent()
diff --git a/test/test_iohelpers.py b/test/test_iohelpers.py
index 2ab3981f..b4fe0f86 100644
--- a/test/test_iohelpers.py
+++ b/test/test_iohelpers.py
@@ -22,18 +22,25 @@ from meld.iohelpers import find_shared_parent_path, format_parent_relative_path
         (['/foo/a/', '/foo/b/asd/asd'], '/foo'),
         # Common parent is the root
         (['/foo/a/', '/bar/b/'], '/'),
+        # One path, one missing path
+        (['/foo/a', None], None),
+        # Two paths, one missing path
+        (['/foo/a', None, '/foo/c'], None),
     ],
 )
 def test_find_shared_parent_path(paths, expected_parent):
-    files = [Gio.File.new_for_path(p) for p in paths]
-    print([f.get_path() for f in files])
+    files = [Gio.File.new_for_path(p) if p else None for p in paths]
+    print([f.get_path() if f else repr(f) for f in files])
     parent = find_shared_parent_path(files)
 
     if parent is None:
         assert expected_parent is None
     else:
         print(f'Parent: {parent.get_path()}; expected {expected_parent}')
-        assert parent.equal(Gio.File.new_for_path(expected_parent))
+        if expected_parent is None:
+            assert parent is None
+        else:
+            assert parent.equal(Gio.File.new_for_path(expected_parent))
 
 
 @pytest.mark.parametrize(