Prevent accidentally dropping TexturesDelta (#8356)

We had a ton of issues around `TexturesDelta` that weren't properly
applied because we early-out of some function:
* https://github.com/emilk/egui/pull/8313
* https://github.com/emilk/egui/pull/8250
* https://github.com/emilk/egui/pull/8279

This PR changes texture updates, so that we always store them after
taking them out of `FullOutput` and keep the delta around until it's
actually applied (by passing &mut refs and draining instead of
iterating). So even if we add a new early return somewhere, that can't
break texture updates.

It also optimizes `TexturesDelta::append` by dropping any previous
deltas if there's a new `whole` delta or a `free`.

It also adds a debug assert that any `TexturesDelta` is empty when
dropped, as an additional safeguard in case the bug sneaks back in.

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/egui/src/context.rs

@@ -775,6 +775,7 @@ impl Context {
     ///     ui.label("Hello egui!");
     /// });
     /// // handle full_output
+    /// # full_output.drop_without_applying_deltas();
     /// ```
     #[must_use]
     pub fn run_ui(&self, new_input: RawInput, mut run_ui: impl FnMut(&mut Ui)) -> FullOutput {
@@ -892,6 +893,7 @@ impl Context {
     ///
     /// let full_output = ctx.end_pass();
     /// // handle full_output
+    /// # full_output.drop_without_applying_deltas();
     /// ```
     pub fn begin_pass(&self, mut new_input: RawInput) {
         profiling::function_scope!();
@@ -4296,6 +4298,7 @@ mod test {
             assert_eq!(num_calls, 1);
             assert_eq!(output.platform_output.num_completed_passes, 1);
             assert!(!output.platform_output.requested_discard());
+            output.drop_without_applying_deltas();
         }
 
         // A single call, with a denied request to discard:
@@ -4321,6 +4324,7 @@ mod test {
                     .reason,
                 "test"
             );
+            output.drop_without_applying_deltas();
         }
     }
 
@@ -4341,6 +4345,7 @@ mod test {
             assert_eq!(num_calls, 1);
             assert_eq!(output.platform_output.num_completed_passes, 1);
             assert!(!output.platform_output.requested_discard());
+            output.drop_without_applying_deltas();
         }
 
         // Request discard once:
@@ -4363,6 +4368,7 @@ mod test {
                 !output.platform_output.requested_discard(),
                 "The request should have been cleared when fulfilled"
             );
+            output.drop_without_applying_deltas();
         }
 
         // Request discard twice:
@@ -4387,6 +4393,7 @@ mod test {
                 output.platform_output.requested_discard(),
                 "The unfulfilled request should be reported"
             );
+            output.drop_without_applying_deltas();
         }
     }
 
@@ -4415,6 +4422,7 @@ mod test {
                 !output.platform_output.requested_discard(),
                 "The request should have been cleared when fulfilled"
             );
+            output.drop_without_applying_deltas();
         }
     }
 }

crates/egui/src/data/output.rs

@@ -16,7 +16,7 @@ pub struct FullOutput {
 
     /// Texture changes since last frame (including the font texture).
     ///
-    /// The backend needs to apply [`crate::TexturesDelta::set`] _before_ painting,
+    /// The backend needs to apply [`crate::TexturesDelta::push`] _before_ painting,
     /// and free any texture in [`crate::TexturesDelta::free`] _after_ painting.
     ///
     /// It is assumed that all egui viewports share the same painter and texture namespace.
@@ -68,6 +68,12 @@ impl FullOutput {
             }
         }
     }
+
+    /// [`epaint::textures::TexturesDelta`] will panic when dropped with still unapplied deltas,
+    /// this is a helper to clear the deltas.
+    pub fn drop_without_applying_deltas(mut self) {
+        self.textures_delta.clear();
+    }
 }
 
 /// Information about text being edited.

crates/egui/src/lib.rs

@@ -673,18 +673,20 @@ pub enum WidgetType {
 pub fn __run_test_ctx(mut run_ui: impl FnMut(&Context)) {
     let ctx = Context::default();
     ctx.set_fonts(FontDefinitions::empty()); // prevent fonts from being loaded (save CPU time)
-    let _ = ctx.run_ui(Default::default(), |ui| {
+    let output = ctx.run_ui(Default::default(), |ui| {
         run_ui(ui.ctx());
     });
+    output.drop_without_applying_deltas();
 }
 
 /// For use in tests; especially doctests.
 pub fn __run_test_ui(mut add_contents: impl FnMut(&mut Ui)) {
     let ctx = Context::default();
     ctx.set_fonts(FontDefinitions::empty()); // prevent fonts from being loaded (save CPU time)
-    let _ = ctx.run_ui(Default::default(), |ui| {
+    let output = ctx.run_ui(Default::default(), |ui| {
         add_contents(ui);
     });
+    output.drop_without_applying_deltas();
 }
 
 pub fn accesskit_root_id() -> Id {

crates/egui/src/text_selection/label_text_selection.rs

@@ -773,7 +773,7 @@ mod tests {
             .or_default()
             .selection = Some(test_selection());
 
-        let _ = ctx.run_ui(RawInput::default(), |_| {});
+        let output = ctx.run_ui(RawInput::default(), |_| {});
         assert!(
             plugin
                 .lock()
@@ -782,11 +782,13 @@ mod tests {
                 .is_some_and(ViewportLabelSelectionState::has_selection),
             "a pass in another viewport must not clear the child viewport selection"
         );
+        output.drop_without_applying_deltas();
 
-        let _ = ctx.run_ui(child_viewport_input(child_viewport_id), |_| {});
+        let output = ctx.run_ui(child_viewport_input(child_viewport_id), |_| {});
         assert!(
             !plugin.lock().has_selection(),
             "the selection must be cleared when its labels disappear from the same viewport"
         );
+        output.drop_without_applying_deltas();
     }
 }

crates/egui/src/viewport.rs

@@ -71,9 +71,8 @@
 
 use std::sync::Arc;
 
-use epaint::{Pos2, Vec2};
-
 use crate::{AsId, Context, Id, Ui};
+use epaint::{Pos2, Vec2};
 
 // ----------------------------------------------------------------------------