Make submenu positioning more reliable

README.md

@@ -137,7 +137,8 @@ cat samples/ship.ans
 - Pressing both mouse buttons stops the current tool, but doesn't undo the current action.
 - Due to limitations of the terminal, shortcuts using Shift or Alt might not work.
 - Menus are not keyboard navigable.
-- Clicking the Zoom submenu doesn't always work. You just have to click it a few times before it opens. <!--It tries to position itself based on its size, but its size isn't known until it's opened — at least, not reliably.-->
+- The Zoom submenu flickers as it opens, and may not always open in the right place.
+- The canvas flickers when zooming in with the Magnifier tool.
 - Some languages don't display correctly.
 - Large files can make the program very slow, as can magnifying the canvas.
 - The program may crash or freeze up randomly, and there's no auto-save feature.

src/textual_paint/menus.py

@@ -123,6 +123,17 @@ class Menu(Container):
         else:
             self.styles.max_height = self.screen.size.height
 
+            # Needed to recompute self.region
+            # This makes it much more reliable to open and open in the correct spot,
+            # but unfortunately none of these things work to avoid flickering:
+            # self.styles.offset = (0, 0)
+            # self.refresh(layout=True)
+            # self.styles.visibility = "hidden"
+            # self.hidden = True
+            self.app.refresh(layout=True)
+            # self.styles.visibility = "visible"
+            # self.hidden = False
+            # I also tried call_after_refresh for the below.
             rect = parent_menu_item.region
             self.styles.offset = (
                 rect.x - self.region.width if get_direction() == "rtl" else rect.x + rect.width,