Instead of clearing, prevent filling filename input in Open dialog

Prevent the filename input from being populated in the first place,
instead of resetting after its unintentionally populated when the
directory tree is expanded.

Nip it in the bud. Head it off at the pass. Stop it at the source.

It still needs this new flag to be cleared on a timer, but at least this
timer is near the code for the other timers it needs to trigger after.

Explicitly set filename for Save As dialog, which SHOULD be populated.

paint.py

@@ -874,6 +874,9 @@ class PaintApp(App):
     # file modification tracking
     saved_undo_count = 0
 
+    # flag to prevent setting the filename input when initially expanding the directory tree
+    expanding_directory_tree = False
+
     NAME_MAP = {
         # key to button id
     }
@@ -1033,9 +1036,10 @@ class PaintApp(App):
             title=_("Save As"),
             handle_button=handle_button,
         )
+        filename = os.path.basename(self.filename) if self.filename else _("Untitled")
         window.content.mount(
             DirectoryTree(id="save_as_dialog_directory_tree", path="/"),
-            Input(id="save_as_dialog_filename_input", placeholder=_("Filename")),
+            Input(id="save_as_dialog_filename_input", placeholder=_("Filename"), value=filename),
             Button(_("Save"), classes="save submit", variant="primary"),
             Button(_("Cancel"), classes="cancel"),
         )
@@ -1046,6 +1050,7 @@ class PaintApp(App):
     def expand_directory_tree(self, tree: DirectoryTree) -> None:
         """Expand the directory tree to the target directory, either the folder of the open file or the current working directory."""
         # TODO: os.path.normcase, and maybe os.path.samefile check
+        self.expanding_directory_tree = True
         target_dir = (self.filename or os.getcwd()).rstrip(os.path.sep)
         node = tree.root
         def get_node_name(node):
@@ -1067,8 +1072,6 @@ class PaintApp(App):
             else:
                 # Directory or file not found.
                 break
-        # NOTE: There is a 0.02s timer meant to run after the directory tree is expanded,
-        # externally, compounding this mess.
         # Timer is needed to wait for the new nodes to mount, I think.
         # tree.select_node(node)
         self.set_timer(0.01, lambda: tree.select_node(node))
@@ -1088,6 +1091,10 @@ class PaintApp(App):
         # Timer is needed to wait for the new nodes to mount, I think.
         self.set_timer(0.01, lambda: tree.scroll_to_region(tree._get_label_region(node._line), animate=False, top=True))
 
+        def done_expanding():
+            self.expanding_directory_tree = False
+        self.set_timer(0.02, done_expanding)
+    
     def confirm_overwrite(self, filename: str, callback) -> None:
         message = _("%1 already exists.\nDo you want to replace it?").replace("%1", filename)
         def handle_button(button):
@@ -1279,16 +1286,7 @@ class PaintApp(App):
             Button(_("Cancel"), classes="cancel"),
         )
         self.mount(window)
-        # This context manager doesn't work because it's a child widget, not self.
-        # ...and/or because it uses a timer to select the file node in the tree.
-        # I might be able to do prevent on the widget itself, if not for the timer.
-        # with self.prevent(DirectoryTree.FileSelected):
         self.expand_directory_tree(window.content.query_one("#open_dialog_directory_tree"))
-        # Reset the filename input, which gets set to the selected file name.
-        def reset_filename_input():
-            window.content.query_one("#open_dialog_filename_input").value = ""
-        # Ugh, expand_directory_tree uses set_timer already.
-        self.set_timer(0.02, reset_filename_input)
 
     def action_new(self, *, force=False) -> None:
         """Create a new image."""
@@ -1751,7 +1749,8 @@ class PaintApp(App):
         elif event.node.parent:
             self.directory_tree_selected_path = event.node.parent.data.path
             name = os.path.basename(event.node.data.path)
-            self.query_one("#save_as_dialog_filename_input, #open_dialog_filename_input", Input).value = name
+            if not self.expanding_directory_tree:
+                self.query_one("#save_as_dialog_filename_input, #open_dialog_filename_input", Input).value = name
         else:
             self.directory_tree_selected_path = None