Show menu controller from centroid of message cells.

Signal/src/ViewControllers/ConversationView/Cells/OWSMessageCell.m

@@ -1200,7 +1200,11 @@ NS_ASSUME_NONNULL_BEGIN
 
     // We "eagerly" respond when the long press begins, not when it ends.
     if (sender.state == UIGestureRecognizerStateBegan) {
-        CGPoint location = [sender locationInView:self];
+        // Show the menu controller from the centroid of the cell,
+        // not the touch location.  This disambiguates which cell
+        // is the context for the menu if the user presents the menu
+        // controller from the border between two cells.
+        CGPoint location = CGPointMake(self.width * 0.5f, self.height * 0.5f);
         [self showMenuController:location];
     }
 }