Merge pull request 'test(formats): invert the primitives tripwire to pin the fix (#38, option A)' (#40) from fix/38-invert-primitives-tripwire into main

Reviewed-on: #40

crates/sylpheed-formats/tests/ui_prm_primitives_disc.rs

@@ -172,8 +172,29 @@ fn the_title_fade_quad_rests_transparent() {
 ///
 /// This test measures the damage rather than asserting the feature works, so the
 /// number stays honest and moves when the ordering is solved.
+/// With primitives drawn, no build comes out wiped.
+///
+/// 🔴 THIS USED TO ASSERT THE OPPOSITE. Until issue #38 it was a tripwire named
+/// `the_derived_order_puts_primitives_last_and_that_wipes_screens`, asserting
+/// `wiped_on > 0` — that switching primitives on still flattened at least one
+/// screen — with the instruction to turn `include_primitives` on by default the
+/// day it stopped. `53f8345` made that day arrive: a keyless primitive that
+/// would hide the screen is now forced to paint first (`forced_backdrop`,
+/// `docs/re/structures/ui-forced-backdrop.md`), and the 36 one-colour builds
+/// were "wiped by our own sort". The wire tripped on 2026-08-29 and sat red for
+/// 15 days, because CI has no corpus and the suite self-skips there.
+///
+/// #38 chose to keep the default **off** and pin the fix instead. The flatness
+/// proxy stood in for "paint order is solved", and the fix's own record says
+/// it is only partly solved: *"This does **not** make `include_primitives` safe
+/// by default"* (`docs/re/structures/ui-prm-primitives.md`). An unlisted
+/// primitive that does not hide the screen still sorts last.
+///
+/// So this is now the one end-to-end check of the rule: the four tests in
+/// `ui_forced_backdrop_disc.rs` pin its mechanics, and none composes with
+/// primitives on.
 #[test]
-fn the_derived_order_puts_primitives_last_and_that_wipes_screens() {
+fn no_build_is_wiped_with_primitives_on() {
     let Some(root) = disc_root() else {
         eprintln!("SKIP: extracted disc not found (set SYLPHEED_DISC to enable)");
         return;
@@ -221,13 +242,17 @@ fn the_derived_order_puts_primitives_last_and_that_wipes_screens() {
         "the DEFAULT composite wipes {wiped_off} builds — primitives are supposed \
          to be off unless asked for"
     );
-    assert!(
-        wiped_on > 0,
-        "no build is wiped with primitives on — the ordering problem this flag \
-         exists for may be solved, in which case turn it on by default"
+    assert_eq!(
+        wiped_on, 0,
+        "{wiped_on} of {with_prm} builds come out >99% one colour with primitives on — \
+         a primitive that hides the screen is painting on top again. That is \
+         `forced_backdrop` failing to force it first; see \
+         docs/re/structures/ui-forced-backdrop.md and issue #38. Do NOT resolve \
+         this by turning `include_primitives` off in the test: the default is \
+         already off, and this test is the only end-to-end check of the rule."
     );
     eprintln!(
-        "{with_prm} builds draw a visible primitive; with the derived order \
-         {wiped_on} of them come out >99% one colour, {wiped_off} by default"
+        "{with_prm} builds draw a visible primitive; with primitives on \
+         {wiped_on} come out >99% one colour, {wiped_off} by default"
     );
 }