Fix crash when parent viewport is hidden (#8226)

Hey,
this is my fist PR to fix the bug I just published #8225 
I hope it helps. 
I tested the fix with wgpu but not glow.
I must say that I am not very happy to have very similar function
is_viewport_or_descendant_visible in both case.
Let me know if you would prefer that I try to factorize it.

* Closes <https://github.com/emilk/egui/issues/8225>
* [X] I have followed the instructions in the PR template

---------

Co-authored-by: Matthieu Casanova <public@kpouer.com>

crates/eframe/src/native/glow_integration.rs

@@ -558,7 +558,7 @@ impl GlowWinitRunning<'_> {
             }
         }
 
-        let (raw_input, viewport_ui_cb, is_visible) = {
+        let (raw_input, viewport_ui_cb, is_visible, run_ui) = {
             let mut glutin = self.glutin.borrow_mut();
             let egui_ctx = glutin.egui_ctx.clone();
             let Some(viewport) = glutin.viewports.get_mut(&viewport_id) else {
@@ -577,6 +577,9 @@ impl GlowWinitRunning<'_> {
             let mut raw_input = egui_winit.take_egui_input(window);
             let viewport_ui_cb = viewport.viewport_ui_cb.clone();
 
+            let run_ui =
+                is_visible || is_viewport_or_descendant_visible(&glutin.viewports, viewport_id);
+
             self.integration.pre_update();
 
             raw_input.time = Some(self.integration.beginning.elapsed().as_secs_f64());
@@ -586,7 +589,7 @@ impl GlowWinitRunning<'_> {
                 .map(|(id, viewport)| (*id, viewport.info.clone()))
                 .collect();
 
-            (raw_input, viewport_ui_cb, is_visible)
+            (raw_input, viewport_ui_cb, is_visible, run_ui)
         };
 
         // HACK: In order to get the right clear_color, the system theme needs to be set, which
@@ -641,7 +644,7 @@ impl GlowWinitRunning<'_> {
             self.app.as_mut(),
             viewport_ui_cb.as_deref(),
             raw_input,
-            is_visible,
+            run_ui,
         );
 
         // ------------------------------------------------------------
@@ -1462,6 +1465,28 @@ fn initialize_or_update_viewport(
     }
 }
 
+/// Is this viewport, or any of its (transitive) descendant viewports, visible?
+///
+/// Immediate viewports are rendered inline while their parent's UI runs, so even
+/// if this viewport's window is occluded or minimized we must still run its UI to
+/// give any visible descendant a chance to be painted.
+fn is_viewport_or_descendant_visible(
+    viewports: &OrderedViewportIdMap<Viewport>,
+    viewport_id: ViewportId,
+) -> bool {
+    let Some(viewport) = viewports.get(&viewport_id) else {
+        return false;
+    };
+    if viewport.info.visible().unwrap_or(true) {
+        return true;
+    }
+    viewports.values().any(|child| {
+        child.ids.parent == viewport_id
+            && child.ids.this != viewport_id // ROOT is its own parent; avoid self-recursion.
+            && is_viewport_or_descendant_visible(viewports, child.ids.this)
+    })
+}
+
 /// This is called (via a callback) by user code to render immediate viewports,
 /// i.e. viewport that are directly nested inside a parent viewport.
 fn render_immediate_viewport(

crates/eframe/src/native/wgpu_integration.rs

@@ -601,7 +601,7 @@ impl WgpuWinitRunning<'_> {
         let mut frame_timer = crate::stopwatch::Stopwatch::new();
         frame_timer.start();
 
-        let (viewport_ui_cb, raw_input, is_visible) = {
+        let (viewport_ui_cb, raw_input, is_visible, run_ui) = {
             profiling::scope!("Prepare");
             let mut shared_lock = shared.borrow_mut();
 
@@ -657,6 +657,8 @@ impl WgpuWinitRunning<'_> {
             };
             let mut raw_input = egui_winit.take_egui_input(window);
 
+            let run_ui = is_visible || is_viewport_or_descendant_visible(viewports, viewport_id);
+
             integration.pre_update();
 
             raw_input.time = Some(integration.beginning.elapsed().as_secs_f64());
@@ -667,19 +669,15 @@ impl WgpuWinitRunning<'_> {
 
             painter.handle_screenshots(&mut raw_input.events);
 
-            (viewport_ui_cb, raw_input, is_visible)
+            (viewport_ui_cb, raw_input, is_visible, run_ui)
         };
 
         // ------------------------------------------------------------
 
         // Runs the update, which could call immediate viewports,
         // so make sure we hold no locks here!
-        let full_output = integration.update(
-            app.as_mut(),
-            viewport_ui_cb.as_deref(),
-            raw_input,
-            is_visible,
-        );
+        let full_output =
+            integration.update(app.as_mut(), viewport_ui_cb.as_deref(), raw_input, run_ui);
 
         // ------------------------------------------------------------
 
@@ -1026,6 +1024,25 @@ fn create_window(
     Ok((window, viewport_builder))
 }
 
+/// Is this viewport, or any of its (transitive) descendant viewports, visible?
+///
+/// Immediate viewports are rendered inline while their parent's UI runs, so even
+/// if this viewport's window is occluded or minimized we must still run its UI to
+/// give any visible descendant a chance to be painted.
+fn is_viewport_or_descendant_visible(viewports: &Viewports, viewport_id: ViewportId) -> bool {
+    let Some(viewport) = viewports.get(&viewport_id) else {
+        return false;
+    };
+    if viewport.info.visible().unwrap_or(true) {
+        return true;
+    }
+    viewports.values().any(|child| {
+        child.ids.parent == viewport_id
+            && child.ids.this != viewport_id
+            && is_viewport_or_descendant_visible(viewports, child.ids.this)
+    })
+}
+
 fn render_immediate_viewport(
     beginning: Instant,
     shared: &RefCell<SharedState>,