X11: Use bottom-right corner of IME cursor area as caret position.

XIM servers currently do not support preedit area reporting from
clients and there may be no standard way to report it.

Fcitx and iBus both place the candidate window descending descending
from the caret, with the reported X font; but since winit does not
report a font, the height of the line is assumed 0, so when we report
the top left corner of the cursor area they will tend to obscure it.

Taking this into account, the best default option is to report the
bottom right corner of the cursor area, because it will tend not to
obscure the preedit area when using `Window::set_ime_cursor_area` in
the way suggested by documentation.

src/changelog/unreleased.md

@@ -125,6 +125,7 @@ changelog entry.
   - `Window::request_inner_size` to `request_surface_size`.
   - `Window::set_min_inner_size` to `set_min_surface_size`.
   - `Window::set_max_inner_size` to `set_max_surface_size`.
+- On X11, report the bottom right corner of the provided cursor area as the caret position, instead of the top left corner. This generally improves behavior with Fcitx and iBus, which both place the candidate window down and two the right from the reported position. Winit programs that already work around this by only reporting position with zero size don't need to be changed.
 
   To migrate, you can probably just replace all instances of `inner_size` with `surface_size` in your codebase.
 - Every event carrying a `DeviceId` now uses `Option<DeviceId>` instead. A `None` value signifies that the

src/platform_impl/linux/x11/window.rs

@@ -2032,12 +2032,19 @@ impl UnownedWindow {
     }
 
     #[inline]
-    pub fn set_ime_cursor_area(&self, spot: Position, _size: Size) {
-        let (x, y) = spot.to_physical::<i32>(self.scale_factor()).into();
+    pub fn set_ime_cursor_area(&self, spot: Position, size: Size) {
+        let PhysicalPosition { x, y } = spot.to_physical::<i16>(self.scale_factor());
+        let PhysicalSize { width, height } = size.to_physical::<i16>(self.scale_factor());
+        // We only currently support reporting a caret position via XIM.
+        // No IM servers currently process preedit area information from XIM clients
+        // and it is unclear this is even part of the standard protocol.
+        // Fcitx and iBus both assume that the position reported is at the insertion
+        // caret, and by default will place the candidate window under and to the
+        // right of the reported point.
         let _ = self.ime_sender.lock().unwrap().send(ImeRequest::Position(
             self.xwindow as ffi::Window,
-            x,
-            y,
+            x.saturating_add(width),
+            y.saturating_add(height),
         ));
     }
 

src/window.rs

@@ -1012,7 +1012,8 @@ pub trait Window: AsAny + Send + Sync {
     ///
     /// ## Platform-specific
     ///
-    /// - **X11:** - area is not supported, only position.
+    /// - **X11:** Area is not supported, only position. The bottom-right corner of the provided
+    ///   area is reported as the position.
     /// - **iOS / Android / Web / Orbital:** Unsupported.
     ///
     /// [chinese]: https://support.apple.com/guide/chinese-input-method/use-the-candidate-window-cim12992/104/mac/12.0