fix(layout): derive available grid space per CSS Grid §11.3 to restore indefinite fr resolution

Author: TtTRz

float-pigment-forest/tests/custom/css_grid/available_grid_space.rs

@@ -0,0 +1,91 @@
+// Regression tests for CSS Grid §11.3 "available grid space".
+//
+// Per spec (https://www.w3.org/TR/css-grid-1/#algo-terms), independently
+// per dimension, the available grid space is:
+//   - the container's content box, when the container has a definite size;
+//   - the active min-/max-content constraint (indefinite), when the
+//     container is being sized under such a constraint;
+//   - indefinite, otherwise.
+//
+// When available grid space is indefinite, fr tracks must be resolved via
+// §11.7.1 ("Find the Size of an fr") using items' max-content contributions,
+// not the ancestor's available space.
+//
+// These tests lock down that the grid container never inherits the
+// ancestor's available space as its own definite available grid space
+// in `SizingMode::Normal`, and therefore the container collapses to the
+// children's contribution under indefinite sizing.
+
+use crate::*;
+
+// §11.3 branch: container size is indefinite (inline-grid, width auto),
+// parent provides available space but it MUST NOT be taken as definite
+// available grid space. fr tracks must resolve via §11.7.1.
+// Expected: container width = sum of children max-content (100 + 100 = 200).
+#[test]
+fn grid_available_space_indefinite_inline_grid_collapses_to_children() {
+    assert_xml!(
+        r#"
+        <div style="display: inline-grid; grid-template-columns: 1fr 1fr;" expect_width="200">
+          <div style="width: 100px; height: 50px;" expect_left="0" expect_width="100"></div>
+          <div style="width: 100px; height: 50px;" expect_left="100" expect_width="100"></div>
+        </div>
+    "#,
+        true
+    )
+}
+
+// §11.3 branch: container has a definite size, available grid space is
+// the content box. fr tracks resolve via the definite path and split it.
+#[test]
+fn grid_available_space_definite_uses_content_box() {
+    assert_xml!(
+        r#"
+        <div style="display: grid; width: 300px; grid-template-columns: 1fr 1fr;">
+          <div style="height: 50px;" expect_left="0" expect_width="150"></div>
+          <div style="height: 50px;" expect_left="150" expect_width="150"></div>
+        </div>
+    "#,
+        true
+    )
+}
+
+// §11.3 branch: indefinite container with unequal fr values still uses
+// §11.7.1 indefinite resolution (based on each child's max-content
+// contribution scaled by its fr): hyp_1fr = max(100/1, 150/2) = 100,
+// so track widths are 1fr=100, 2fr=200, and container width = 300.
+// This test asserts the container width only; item-in-track sizing
+// (§11.7/§11.8) is intentionally not asserted here.
+#[test]
+fn grid_available_space_indefinite_unequal_fr_uses_max_contribution() {
+    assert_xml!(
+        r#"
+        <div style="display: inline-grid; grid-template-columns: 1fr 2fr;" expect_width="300">
+          <div style="width: 100px; height: 50px;"></div>
+          <div style="width: 150px; height: 50px;"></div>
+        </div>
+    "#,
+        true
+    )
+}
+
+// Nested case: a block flow wraps the inline-grid. The outer ancestor has
+// a definite width (375 default viewport) and propagates available space
+// down; if that value leaks into the inner grid's available grid space,
+// fr tracks would wrongly split it. This test pins the regression fixed
+// in grid/mod.rs: in Normal sizing mode, `request.max_content` of the
+// inner grid MUST NOT be used as its available grid space.
+#[test]
+fn grid_available_space_indefinite_ignores_ancestor_available_space() {
+    assert_xml!(
+        r#"
+        <div>
+          <div style="display: inline-grid; grid-template-columns: 1fr 1fr;" expect_width="200">
+            <div style="width: 100px; height: 50px;" expect_left="0" expect_width="100"></div>
+            <div style="width: 100px; height: 50px;" expect_left="100" expect_width="100"></div>
+          </div>
+        </div>
+    "#,
+        true
+    )
+}

float-pigment-forest/tests/custom/css_grid/mod.rs

@@ -5,6 +5,7 @@
 // - Grid items are placed into cells according to the grid auto-placement algorithm
 // - `auto` track sizing allows tracks to size based on content
 
+mod available_grid_space;
 mod gap;
 
 use crate::*;

float-pigment-layout/src/algo/grid/mod.rs

@@ -164,10 +164,18 @@ impl<T: LayoutTreeNode> GridContainer<T> for LayoutUnit<T> {
             requested_size.width - padding_border.horizontal(),
             requested_size.height - padding_border.vertical(),
         ));
-
+        // CSS Grid §11.3 "available grid space" (per dimension):
+        //   - Definite container size => use its content box.
+        //   - Under a min-/max-content constraint => use that constraint
+        //     (still indefinite).
+        //   - Otherwise => indefinite.
+        let available_grid_space_source = match request.sizing_mode {
+            SizingMode::Normal => *requested_size,
+            SizingMode::MinContent | SizingMode::MaxContent => *request.max_content,
+        };
         let mut available_grid_space = OptionSize::new(
-            requested_size.width.or(request.max_content.width) - padding_border.horizontal(),
-            requested_size.height.or(request.max_content.height) - padding_border.vertical(),
+            available_grid_space_source.width - padding_border.horizontal(),
+            available_grid_space_source.height - padding_border.vertical(),
         );
 
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