Tests: one wgpu device compiles shaders at a time in view.rs (WARP ARM64 crash)

On GitHub's Windows 11 ARM64 runner (no GPU, so wgpu uses Microsoft's WARP)
tests/view.rs died with STATUS_ACCESS_VIOLATION in about 1 run in 4. A crash
dump puts it inside WARP's pixel-shader JIT
(d3d10warp!PixelJitProgram::ClassifyVars, on WARP's own thread pool) when the
two tests that render pixels compile shaders on separate devices at once.
Single-threaded, or each test alone, it never crashed (180 runs); bigger
thread stacks didn't help (6/30 vs 7/30), so it's that race, not our code.
The app has one device on a real GPU and can't hit it.

Those two tests now hold a per-binary lock for their whole run (taken before
the harness, so released after its device drops). Other test binaries render
from at most one test.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/tests/view.rs

@@ -15,6 +15,18 @@ const PAGES: &[u8] = b"%PDF-1.7
 trailer << /Root 1 0 R >>
 %%EOF";
 
+/// Tests that render pixels (`h.render()`) take this first, so only one wgpu device compiles
+/// shaders at a time. On machines without a GPU, wgpu falls back to Microsoft's WARP, whose ARM64
+/// pixel-shader JIT crashes (access violation in `d3d10warp!PixelJitProgram::ClassifyVars`) when
+/// two devices compile at once, as on GitHub's Windows 11 ARM64 runner. Hold it for the whole
+/// test (declared before the harness, so it's released after the device is dropped).
+static GPU: std::sync::Mutex<()> = std::sync::Mutex::new(());
+
+fn gpu() -> std::sync::MutexGuard<'static, ()> {
+    // A test that panicked while holding it leaves nothing to clean up.
+    GPU.lock().unwrap_or_else(std::sync::PoisonError::into_inner)
+}
+
 fn harness(options: &'static [(&'static str, &'static str)]) -> Harness<'static, PrintCraftApp> {
     let mut h = Harness::builder().with_size(egui::vec2(1400.0, 900.0)).build_eframe(move |_cc| {
         let mut app = PrintCraftApp::new();
@@ -145,6 +157,7 @@ fn field_list_jumps_to_a_fields_page() {
 #[test]
 fn highlight_fields_tints_the_field_area() {
     use egui_kittest::kittest::Queryable;
+    let _gpu = gpu();
     let mut h = form_harness();
     h.state_mut().set_option("panel", "none").unwrap();
     for _ in 0..100 {
@@ -204,6 +217,7 @@ fn the_hand_tool_pans_by_dragging() {
 
 #[test]
 fn required_fields_get_a_red_border_when_highlighting() {
+    let _gpu = gpu();
     let mut h = form_harness();
     h.state_mut().set_option("panel", "none").unwrap();
     h.state_mut().set_option("fields", "on").unwrap();