Tests: harnesses that render through wgpu take turns (#5354)

Two wgpu devices drawing at once crash Windows' software rasterizer (WARP, d3d10warp.dll: 0xc0000005, sometimes heap corruption 0xc0000374), which is the adapter a machine without a GPU or a remote session gets. The ui-egui tests that render (a dragged shape, a rotation drag, and the ignored spelling review preview) now hold one lock while their harness lives. On Windows 11 over SSH the two rendering tests crashed 8 of 20 runs together (0 of 20 alone), and the whole ui-egui lib run 2 of 6; with the lock 0 of 20 and 0 of 10.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/objects/tests.rs

@@ -563,7 +563,8 @@ fn an_inline_picture_drags_to_the_insertion_cursor_and_stays_inline() {
 /// page: its preview has its red outline and nothing dark inside.
 #[test]
 fn a_dragged_shape_leaves_the_text_behind_it() {
-    use crate::table_resize::tests::{H, button, steps};
+    use crate::table_resize::tests::{H, button, gpu_turn, steps};
+    let _gpu = gpu_turn();
     let text = "MMMMMMMMMMMM ".repeat(12);
     let mut a = WordApp::new(wordcraft_engine::Session::new(Document::from_text(&text)), Default::default());
     let shape = InlineObject::Shape {
@@ -758,7 +759,8 @@ fn rotation_window(app: WordApp) -> crate::table_resize::tests::H {
 /// pictures whose cached preview already contains a rotation and either reflection.
 #[test]
 fn rotation_drag_turns_picture_pixels_live_and_commits_once_or_cancels() {
-    use crate::table_resize::tests::{H, button, steps};
+    use crate::table_resize::tests::{H, button, gpu_turn, steps};
+    let _gpu = gpu_turn();
     for (initial, flip_h, flip_v) in [(0.0, false, false), (30.0, true, false), (90.0, false, true), (315.0, true, true)] {
         // Four original colour quadrants make both accidental extra rotation and reflected
         // UVs observable. Sample near the long ends after a quarter turn, outside the old

crates/ui-egui/src/proofing_pane.rs

@@ -373,6 +373,7 @@ mod tests {
     #[test]
     #[ignore = "offscreen visual review; requires a software or hardware graphics adapter"]
     fn spelling_review_dialog_preview() {
+        let _gpu = crate::table_resize::tests::gpu_turn();
         let mut a = app("The quick brwn fox.");
         a.run("review.spelling", json!({})).unwrap();
         let mut h = egui_kittest::Harness::builder().with_size(egui::vec2(1000.0, 700.0)).wgpu().build_ui_state(frame, a);

crates/ui-egui/src/table_resize/tests.rs

@@ -65,6 +65,14 @@ pub(crate) fn steps(h: &mut H, n: usize) {
     }
 }
 
+/// Held by a test while its harness renders through wgpu: two devices drawing at once crash
+/// Windows' software rasterizer (WARP, `d3d10warp.dll`), which is what a machine without a GPU
+/// or a remote session gets, so those tests take turns.
+pub(crate) fn gpu_turn() -> std::sync::MutexGuard<'static, ()> {
+    static GPU: std::sync::Mutex<()> = std::sync::Mutex::new(());
+    GPU.lock().unwrap_or_else(std::sync::PoisonError::into_inner)
+}
+
 /// A real mouse drag of a column border: the resize arrow on hover, a preview and no change
 /// while dragging, then one undo step on release, the selection untouched.
 #[test]