Unconstrain the IME popup when it appeared, resized, requested to

repositioned

- places the IME popup correctly.
- adjusts its position to considering the output (screen) rect.
  - offset if right edge overflows
  - flip vertically if bottom edge overflows

src/wayland/handlers/xdg_shell/popup.rs

@@ -4,7 +4,7 @@ use crate::{shell::Shell, utils::prelude::*};
 use smithay::{
     desktop::{
         LayerSurface, PopupKind, PopupManager, WindowSurfaceType, get_popup_toplevel_coords,
-        layer_map_for_output, space::SpaceElement,
+        layer_map_for_output, space::SpaceElement, utils,
     },
     output::Output,
     reexports::{
@@ -175,7 +175,33 @@ fn position_popup_within_rect(
             });
             rect.contains_rect(geometry)
         }
-        PopupKind::InputMethod(_) => false,
+        PopupKind::InputMethod(popup) => {
+            let input_rect = popup.text_input_rectangle();
+
+            // We basically place the IME popup below the input rect.
+            let mut popup_bbox = utils::bbox_from_surface_tree(popup.wl_surface(), input_rect.loc);
+            popup_bbox.loc.y += input_rect.size.h;
+            // tracing::debug!(
+            //     "IME input_rect: {:?}, popup_bbox: {:?}",
+            //     input_rect,
+            //     popup_bbox
+            // );
+
+            // Handle the right edge overflow
+            let popup_right = popup_bbox.loc.x + popup_bbox.size.w;
+            let rect_right = rect.loc.x + rect.size.w;
+            popup_bbox.loc.x -= (popup_right - rect_right).max(0);
+
+            // Flip vertically if the bottom edge overflows
+            let popup_bottom = popup_bbox.loc.y + popup_bbox.size.h;
+            let rect_bottom = rect.loc.y + rect.size.h;
+            if popup_bottom > rect_bottom {
+                popup_bbox.loc.y = input_rect.loc.y - popup_bbox.size.h;
+            }
+
+            popup.set_location(popup_bbox.loc);
+            rect.as_logical().contains_rect(popup_bbox)
+        }
     }
 }