Delegate handling of IME interruptions to integrations to fix virtual keyboard flickering on web (#8078)

* Closes N/A
* Partially replaces #7983
* Related: #8045
* [x] I have followed the instructions in the PR template

## Details

In #7983, I modified `Memory::request_focus` to interrupt any ongoing
IME composition. This fixed a bug where clicking inside an already
focused `TextEdit` failed to cancel the active composition, resulting in
duplicated text:

https://github.com/emilk/egui/pull/8045#issuecomment-4193310616

To avoid introducing API changes in that PR, I ensured the IME state was
reset by forcing `PlatformOutput::ime` to `None` for at least one frame.
While this works well on desktop platforms, it causes virtual keyboard
flickering on the web:

https://github.com/emilk/egui/pull/8045#issuecomment-4193035008

In this PR, I delegate the responsibility for handling IME composition
interruptions to integrations, allowing each integration to decide how
to interrupt compositions in a flexible manner.

### The new field `should_interrupt_composition` on `IMEOutput`.

Instead of introducing a new `OutputCommand` variant, this PR adds a new
field `should_interrupt_composition` to `IMEOutput`.

Interrupting an active composition is only meaningful when IME remains
allowed. If IME should be disabled altogether, `PlatformOutput::ime` can
simply be set to `None`.
Given this, IMO, it is more appropriate to attach the interrupt signal
to `IMEOutput` (i.e., the type of `PlatformOutput::ime`).

crates/egui/src/context.rs

@@ -2612,6 +2612,12 @@ impl ContextImpl {
 
         let mut platform_output: PlatformOutput = std::mem::take(&mut viewport.output);
 
+        if self.memory.should_interrupt_ime()
+            && let Some(ime) = &mut platform_output.ime
+        {
+            ime.should_interrupt_composition = true;
+        }
+
         {
             profiling::scope!("accesskit");
             let state = viewport.this_pass.accesskit_state.take();

crates/egui/src/data/output.rs

@@ -79,6 +79,9 @@ pub struct IMEOutput {
     ///
     /// This is a very thin rectangle.
     pub cursor_rect: crate::Rect,
+
+    /// Whether any ongoing IME composition should be interrupted.
+    pub should_interrupt_composition: bool,
 }
 
 /// Commands that the egui integration should execute at the end of a frame.

crates/egui/src/memory/mod.rs

@@ -117,21 +117,10 @@ pub struct Memory {
     #[cfg_attr(feature = "persistence", serde(skip))]
     popups: ViewportIdMap<OpenPopup>,
 
-    /// When the last IME interruption was made.
+    /// Whether to inform the backend to interrupt any ongoing IME composition
+    /// this pass.
     #[cfg_attr(feature = "persistence", serde(skip))]
-    ime_interruption_time: ImeInterruptionTime,
-}
-
-#[derive(Clone, Copy, Debug, Default)]
-enum ImeInterruptionTime {
-    #[default]
-    None,
-
-    /// The IME was interrupted in the current frame.
-    ThisFrame,
-
-    /// The IME was interrupted in the previous frame.
-    LastFrame,
+    requested_interrupt_ime: bool,
 }
 
 impl Default for Memory {
@@ -149,7 +138,7 @@ impl Default for Memory {
             popups: Default::default(),
             everything_is_visible: Default::default(),
             add_fonts: Default::default(),
-            ime_interruption_time: Default::default(),
+            requested_interrupt_ime: Default::default(),
         };
         slf.interactions.entry(slf.viewport_id).or_default();
         slf.areas.entry(slf.viewport_id).or_default();
@@ -778,15 +767,7 @@ impl Memory {
 
         self.areas.entry(self.viewport_id).or_default();
 
-        match self.ime_interruption_time {
-            ImeInterruptionTime::ThisFrame => {
-                self.ime_interruption_time = ImeInterruptionTime::LastFrame;
-            }
-            ImeInterruptionTime::LastFrame => {
-                self.ime_interruption_time = ImeInterruptionTime::None;
-            }
-            ImeInterruptionTime::None => {}
-        }
+        self.requested_interrupt_ime = false;
 
         // self.interactions  is handled elsewhere
 
@@ -1028,30 +1009,22 @@ impl Memory {
     ///
     /// A widget should only consume IME events if this returns `true`. At most
     /// one widget can own IME events for each frame.
+    #[inline(always)]
     pub fn owns_ime_events(&self, id: Id) -> bool {
-        let Some(focus) = self.focus() else {
-            return false;
-        };
-        // We check across two frames because the widget that called
-        // `interrupt_ime` may run after other widgets that call this method
-        // within the same frame.
-        if matches!(
-            self.ime_interruption_time,
-            ImeInterruptionTime::ThisFrame | ImeInterruptionTime::LastFrame
-        ) {
-            return false;
-        }
-        focus.focused() == Some(id)
+        // Note: Even if the IME is being interrupted in the current frame, we
+        // should not return `false` here, since we still need
+        // `PlatformOutput::ime` to be set in such cases.
+
+        self.has_focus(id)
     }
 
     /// Interrupt the current IME composition, if any.
-    ///
-    /// This causes [`Self::owns_ime_events`] to return `false` for all widgets
-    /// for the remainder of this frame and the next frame, giving time
-    /// for the IME to be dismissed (by making `platform_output.ime` be `None`
-    /// for at least one frame).
     pub fn interrupt_ime(&mut self) {
-        self.ime_interruption_time = ImeInterruptionTime::ThisFrame;
+        self.requested_interrupt_ime = true;
+    }
+
+    pub(crate) fn should_interrupt_ime(&self) -> bool {
+        self.requested_interrupt_ime
     }
 }
 

crates/egui/src/widgets/text_edit/builder.rs

@@ -871,6 +871,7 @@ impl TextEdit<'_> {
                             o.ime = Some(crate::output::IMEOutput {
                                 rect: to_global * inner_rect,
                                 cursor_rect: to_global * primary_cursor_rect,
+                                should_interrupt_composition: false,
                             });
                         });
                     }