Fix lack of undo state for paste+stamp

README.md

@@ -182,7 +182,6 @@ To preview ANSI art files in file managers like Nautilus, Thunar, Nemo, or Caja,
 - Status bar description can be left blank when selecting a menu item <!--I'm guessing Leave can come after close-->
 - Menu items like Copy/Cut/Paste are not grayed out when inapplicable. Only unimplemented items are grayed out.
 - Set As Wallpaper may not work on your system. For me, on Ubuntu, the wallpaper setting is updated but the picture is not, unless I manually pick it. There is however untested support for many platforms, and you may have better luck than me.
-- If you paste and then stamp the selection with Ctrl+Click, the stamp(s) can't be undone. An undo state is only created when finalizing the selection, for pasted selections.
 - ANSI files (.ans) are treated as UTF-8 when saving and loading, rather than CP437 or Windows-1252 or any other encodings. Unicode is nice and modern terminals support it, but it's not the standard for ANSI files. There isn't really a standard for ANSI files.
 - ANSI files are loaded with a white background. This may make sense as a default for text files, but ANSI files either draw a background or assume a black background, being designed for terminals.
 - Can click Cancel button of Edit Colors dialog while opening it, if the mouse lines up with it.

src/textual_paint/paint.py

@@ -3964,7 +3964,23 @@ Columns: {len(palette) // 2}
                     # Already cut out, don't replace the image data.
                     # But if you hold Ctrl, stamp the selection.
                     if event.ctrl:
+                        # If pasted, it needs an undo state.
+                        # Otherwise, one should have been already created.
+                        if sel.pasted:
+                            sel.pasted = False # don't create undo when melding (TODO: rename flag or refactor)
+
+                            action = Action("Paste")
+                            if len(self.redos) > 0:
+                                self.redos = []
+                            self.undos.append(action)
+                            # The region must be the whole canvas, because when the selection
+                            # is melded with the canvas, it could be anywhere.
+                            # This could be optimized, see extract_to_selection.
+                            action.region = Region(0, 0, self.image.width, self.image.height)
+                            action.update(self.image)
                         sel.copy_to_document(self.image)
+                        # Don't need to refresh canvas since selection occludes the affected region,
+                        # and has the same content anyway, being a stamp.
                     return
                 self.extract_to_selection(not event.ctrl)
                 return