Cancel active tool by pressing both mouse buttons

README.md

@@ -128,7 +128,8 @@ cat file.ans
 
 - Pressing Delete while using the Text tool deletes the whole text box. Similarly, Ctrl+A selects the canvas instead of the text in the text box.
 - Selection box border is inside instead of outside (and lacks dashes). For the text box, I hid the border because it was too visually confusing, but it should also have an outer border.
-- Pick Color can't be cancelled, since it samples the color continuously. No tools can be cancelled by pressing both mouse buttons, only with Escape.
+- Pick Color can't be cancelled, since it samples the color continuously.
+- Pressing both mouse buttons to cancel tools can sometimes result the the tool restarting, or it's just my mouse. It's probably my mouse, now that I think about it. But also, it should actually undo current action, not just end it.
 - Help > Help Topics isn't very helpful.
 - Due to limitations of the terminal, shortcuts using Shift or Alt might not work.
 - Menus are not keyboard navigable.

paint.py

@@ -944,14 +944,21 @@ class Canvas(Widget):
         self.pointer_active: bool = False
         self.magnifier_preview_region: Optional[Region] = None
         self.select_preview_region: Optional[Region] = None
+        self.which_button: Optional[int] = None
 
     def on_mouse_down(self, event: events.MouseDown) -> None:
         self.fix_mouse_event(event) # not needed, pointer isn't captured yet.
         event.x //= self.magnification
         event.y //= self.magnification
 
+        if self.pointer_active and self.which_button != event.button:
+            assert isinstance(self.app, PaintApp)
+            self.app.stop_action_in_progress()
+            return
+
         self.post_message(self.ToolStart(event))
         self.pointer_active = True
+        self.which_button = event.button
         self.capture_mouse(True)
     
     def fix_mouse_event(self, event: events.MouseEvent) -> None: