windows: fix incorrect cursor_range calculation in Ime::Preedit

The `text` is retrieved as UTF-8 while `attributes` are based on UTF-16,
thus the offset was getting out of sync on some unicode payloads
like surrogate pairs.

Fixes #3967.

src/changelog/unreleased.md

@@ -257,3 +257,4 @@ changelog entry.
 - On macOS, fixed `VideoMode::refresh_rate_millihertz` for fractional refresh rates.
 - On macOS, store monitor handle to avoid panics after going in/out of sleep.
 - On macOS, allow certain invalid monitor handles and return `None` instead of panicking.
+- On Windows, fixed `Ime::Preedit` cursor offset calculation.

src/platform_impl/windows/ime.rs

@@ -34,8 +34,13 @@ impl ImeContext {
         let mut first = None;
         let mut last = None;
         let mut boundary_before_char = 0;
+        let mut attr_idx = 0;
+
+        for chr in text.chars() {
+            let Some(attr) = attrs.get(attr_idx).copied() else {
+                break;
+            };
 
-        for (attr, chr) in attrs.into_iter().zip(text.chars()) {
             let char_is_targeted =
                 attr as u32 == ATTR_TARGET_CONVERTED || attr as u32 == ATTR_TARGET_NOTCONVERTED;
 
@@ -46,6 +51,7 @@ impl ImeContext {
             }
 
             boundary_before_char += chr.len_utf8();
+            attr_idx += chr.len_utf16();
         }
 
         if first.is_some() && last.is_none() {