diff --git a/crates/gpui/src/elements/div/inline.rs b/crates/gpui/src/elements/div/inline.rs
index 9c55d7594b7..6f9dbcb8980 100644
--- a/crates/gpui/src/elements/div/inline.rs
+++ b/crates/gpui/src/elements/div/inline.rs
@@ -127,7 +127,11 @@ impl InlineDocument {
         measurement
     }
 
+    /// Every range is clamped to `candidate.text`'s char boundaries: a source range that ends
+    /// inside a multi-byte character maps to the same split in the output, and the text system
+    /// slices the output text by these ranges.
     fn truncated(&self, candidate: &TruncationCandidate) -> Self {
+        let text = candidate.text.as_ref();
         let text_styles = self
             .text_styles
             .iter()
@@ -135,6 +139,7 @@ impl InlineDocument {
                 candidate
                     .display_ranges(&style.range)
                     .into_iter()
+                    .filter_map(|range| clamp_to_char_boundaries(text, range))
                     .map(|range| InlineTextStyle {
                         range,
                         ..style.clone()
@@ -148,6 +153,7 @@ impl InlineDocument {
                 candidate
                     .display_ranges(&span.text_range)
                     .into_iter()
+                    .filter_map(|range| clamp_to_char_boundaries(text, range))
                     .map(|text_range| InlineSpan {
                         layout_id: span.layout_id,
                         text_range,
@@ -159,7 +165,7 @@ impl InlineDocument {
             .collect();
 
         Self {
-            text: candidate.text.to_string(),
+            text: text.to_string(),
             runs: candidate.runs.clone(),
             text_styles,
             spans,
@@ -168,6 +174,22 @@ impl InlineDocument {
     }
 }
 
+/// Moves both ends of `range` down to a char boundary of `text`, never past its end. Returns
+/// `None` when nothing is left.
+fn clamp_to_char_boundaries(text: &str, range: Range<usize>) -> Option<Range<usize>> {
+    let floor = |index: usize| {
+        let mut index = index.min(text.len());
+        while !text.is_char_boundary(index) {
+            index -= 1;
+        }
+        index
+    };
+    let start = floor(range.start);
+    let end = floor(range.end);
+
+    (start < end).then_some(start..end)
+}
+
 struct InlineParagraphMeasurement {
     truncate_width: Option<Pixels>,
     options: crate::TextLayoutOptions,
@@ -620,3 +642,102 @@ fn merge_fragments(regions: &mut Vec<Bounds<Pixels>>) {
 
     *regions = merged;
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::{TruncateFrom, px};
+
+    /// Truncates `text` to at most `max_chars` chars (marker included) and maps `style_ranges`
+    /// into the result the way a layout pass does.
+    fn truncate(
+        text: &str,
+        style_ranges: &[Range<usize>],
+        direction: TruncateFrom,
+        max_chars: usize,
+    ) -> InlineDocument {
+        let document = InlineDocument {
+            text: text.to_owned(),
+            runs: vec![TextRun {
+                len: text.len(),
+                ..Default::default()
+            }],
+            text_styles: style_ranges
+                .iter()
+                .map(|range| InlineTextStyle {
+                    range: range.clone(),
+                    font_size: px(14.),
+                    line_height: px(20.),
+                })
+                .collect(),
+            ..InlineDocument::default()
+        };
+        let boundaries = text
+            .grapheme_indices(true)
+            .map(|(index, _grapheme)| index)
+            .chain(std::iter::once(text.len()));
+        let (candidate, ()) = truncate_with_measured_candidates(
+            text,
+            boundaries,
+            None,
+            "…",
+            &document.runs,
+            direction,
+            |candidate| ((), candidate.text.chars().count() <= max_chars),
+        );
+
+        document.truncated(&candidate)
+    }
+
+    fn assert_ranges_slice(document: &InlineDocument) {
+        for style in &document.text_styles {
+            assert!(
+                document.text.get(style.range.clone()).is_some(),
+                "{:?} must slice {:?} on char boundaries",
+                style.range,
+                document.text
+            );
+        }
+    }
+
+    #[test]
+    fn clamps_a_range_past_the_end_of_a_short_string() {
+        assert_eq!(clamp_to_char_boundaries("abcd", 2..10), Some(2..4));
+        assert_eq!(clamp_to_char_boundaries("abcd", 5..9), None);
+        assert_eq!(
+            clamp_to_char_boundaries("abcd", 125_780_070_648_069..125_780_070_648_070),
+            None
+        );
+        assert_eq!(clamp_to_char_boundaries("", 0..1), None);
+    }
+
+    #[test]
+    fn clamps_a_range_inside_a_multi_byte_char() {
+        // "é" is bytes 0..2, "🦀" is bytes 2..6.
+        assert_eq!(clamp_to_char_boundaries("é🦀", 1..3), Some(0..2));
+        assert_eq!(clamp_to_char_boundaries("é🦀", 3..5), None);
+        assert_eq!(clamp_to_char_boundaries("é🦀", 0..6), Some(0..6));
+    }
+
+    #[test]
+    fn truncating_a_short_string_keeps_style_ranges_inside_the_output() {
+        for direction in [TruncateFrom::End, TruncateFrom::Start, TruncateFrom::Middle] {
+            let document = truncate("abcd", &[0..4, 2..10, 1_000_000..1_000_005], direction, 3);
+
+            assert_eq!(document.text.chars().count(), 3, "{direction:?}");
+            assert_ranges_slice(&document);
+        }
+    }
+
+    #[test]
+    fn truncating_a_multi_byte_string_keeps_style_ranges_on_char_boundaries() {
+        // 2..8 starts inside "é" and ends inside "🦀"; 7..12 starts inside "🦀".
+        let text = "héllo🦀wörld";
+        for direction in [TruncateFrom::End, TruncateFrom::Start, TruncateFrom::Middle] {
+            let document = truncate(text, &[0..2, 2..8, 7..12, 0..text.len()], direction, 8);
+
+            assert!(document.text.chars().count() <= 8, "{direction:?}");
+            assert_ranges_slice(&document);
+        }
+    }
+}
diff --git a/crates/gpui_windows/src/events.rs b/crates/gpui_windows/src/events.rs
index e14f9aef48b..453012340ac 100644
--- a/crates/gpui_windows/src/events.rs
+++ b/crates/gpui_windows/src/events.rs
@@ -171,7 +171,7 @@ impl WindowsWindowInner {
             WM_SHOWWINDOW => self.handle_window_visibility_changed(handle, wparam),
             WM_GPUI_CURSOR_STYLE_CHANGED => self.handle_cursor_changed(wparam, lparam),
             WM_GPUI_FORCE_UPDATE_WINDOW => self.draw_window(handle, true),
-            WM_GPUI_GPU_DEVICE_LOST => self.handle_device_lost(lparam),
+            WM_GPUI_GPU_DEVICE_LOST => self.handle_device_lost(wparam, lparam),
             DM_POINTERHITTEST => self.handle_dm_pointer_hit_test(wparam),
             WM_GETOBJECT => self.handle_wm_getobject(wparam, lparam),
             _ => None,
@@ -1322,7 +1322,14 @@ impl WindowsWindowInner {
         None
     }
 
-    fn handle_device_lost(&self, lparam: LPARAM) -> Option<isize> {
+    fn handle_device_lost(&self, wparam: WPARAM, lparam: LPARAM) -> Option<isize> {
+        // lparam is read as a `*const DirectXDevices`. `WM_USER + 7` is a common
+        // number that IMEs, overlays and accessibility tools also send, so only
+        // trust a message that carries this app's token.
+        if wparam.0 != self.validation_number {
+            log::error!("Wrong validation number while processing device lost message");
+            return None;
+        }
         let devices = lparam.0 as *const DirectXDevices;
         let devices = unsafe { &*devices };
         if let Err(err) = self
diff --git a/crates/gpui_windows/src/platform.rs b/crates/gpui_windows/src/platform.rs
index 4b96ab993e7..8991b080f0f 100644
--- a/crates/gpui_windows/src/platform.rs
+++ b/crates/gpui_windows/src/platform.rs
@@ -1933,6 +1933,81 @@ mod tests {
         Ok(())
     }
 
+    /// Another process or an injected DLL can send `WM_USER + 7` to a gpui window. The window
+    /// must not read that message's lparam as its `DirectXDevices` pointer unless it carries the
+    /// app's validation number: a null lparam faults at the `device` field, a random one corrupts
+    /// memory.
+    #[test]
+    fn a_foreign_device_lost_message_is_ignored() -> anyhow::Result<()> {
+        use super::WindowsPlatform;
+        use crate::{WM_GPUI_GPU_DEVICE_LOST, WindowsWindow};
+        use gpui::{
+            AnyWindowHandle, Bounds, PlatformWindow as _, WindowHandle, WindowId, WindowKind,
+            WindowParams, point, px, size,
+        };
+        use windows::Win32::Foundation::{LPARAM, WPARAM};
+        use windows::Win32::UI::WindowsAndMessaging::{DestroyWindow, SendMessageW};
+
+        struct Root;
+        impl gpui::Render for Root {
+            fn render(
+                &mut self,
+                _: &mut gpui::Window,
+                _: &mut gpui::Context<Self>,
+            ) -> impl gpui::IntoElement {
+                gpui::Empty
+            }
+        }
+
+        let platform = WindowsPlatform::new(false)?;
+        let validation_number = platform.inner.validation_number;
+        let handle: AnyWindowHandle = WindowHandle::<Root>::new(WindowId::from(1)).into();
+        let window = WindowsWindow::new(
+            handle,
+            WindowParams {
+                bounds: Bounds::new(point(px(100.), px(100.)), size(px(400.), px(300.))),
+                titlebar: None,
+                kind: WindowKind::Normal,
+                is_movable: true,
+                app_owns_titlebar_drag: false,
+                is_resizable: true,
+                is_minimizable: true,
+                focus: false,
+                show: false,
+                icon: None,
+                display_id: None,
+                app_id: None,
+                window_min_size: None,
+                windows_renderer: Default::default(),
+            },
+            platform.generate_creation_info(),
+        )?;
+        let hwnd = window.get_raw_handle();
+        let size_before = window.0.state.renderer.borrow().drawable_size();
+
+        // Neither message carries the token, so neither lparam may be dereferenced. Before the
+        // check, the null one crashed with an access violation at 0x10.
+        for (wparam, lparam) in [(0, 0), (validation_number.wrapping_add(1), 0x10)] {
+            unsafe {
+                SendMessageW(
+                    hwnd,
+                    WM_GPUI_GPU_DEVICE_LOST,
+                    Some(WPARAM(wparam)),
+                    Some(LPARAM(lparam)),
+                );
+            }
+        }
+
+        assert_eq!(
+            window.0.state.renderer.borrow().drawable_size(),
+            size_before,
+            "the renderer must be untouched by a message without the token"
+        );
+
+        unsafe { DestroyWindow(hwnd)? };
+        Ok(())
+    }
+
     #[test]
     fn test_clipboard() {
         let item = ClipboardItem::new_string("你好，我是张小白".to_string());