Improve estimate of grid size

src/compute/grid/implicit_grid.rs

@@ -28,12 +28,12 @@ pub(crate) fn compute_grid_size_estimate<'a>(
     // Compute *track* count estimates for each axis from:
     //   - The explicit track counts
     //   - The origin-zero coordinate min and max grid line variables
-    let negative_implicit_inline_tracks = col_min.unsigned_abs();
+    let negative_implicit_inline_tracks = col_min.implied_negative_implicit_tracks();
     let explicit_inline_tracks = explicit_col_count;
-    let mut positive_implicit_inline_tracks = max(explicit_col_count, col_max.unsigned_abs()) - explicit_col_count;
-    let negative_implicit_block_tracks = row_min.unsigned_abs();
+    let mut positive_implicit_inline_tracks = col_max.implied_positive_implicit_tracks(explicit_col_count);
+    let negative_implicit_block_tracks = row_min.implied_negative_implicit_tracks();
     let explicit_block_tracks = explicit_row_count;
-    let mut positive_implicit_block_tracks = max(explicit_row_count, row_max.unsigned_abs()) - explicit_row_count;
+    let mut positive_implicit_block_tracks = row_max.implied_positive_implicit_tracks(explicit_row_count);
 
     // In each axis, adjust positive track estimate if any items have a span that does not fit within
     // the total number of tracks in the estimate

src/compute/grid/types/coordinates.rs

@@ -96,8 +96,22 @@ impl OriginZeroLine {
         2 * ((self.0 + track_counts.negative_implicit as i16) as usize)
     }
 
-    pub(crate) fn unsigned_abs(self) -> u16 {
-        self.0.unsigned_abs()
+    /// The minimum number of negative implicit track there must be if a grid item starts at this line.
+    pub(crate) fn implied_negative_implicit_tracks(self) -> u16 {
+        if self.0 < 0 {
+            self.0.unsigned_abs()
+        } else {
+            0
+        }
+    }
+
+    /// The minimum number of positive implicit track there must be if a grid item end at this line.
+    pub(crate) fn implied_positive_implicit_tracks(self, explicit_track_count: u16) -> u16 {
+        if self.0 > explicit_track_count as i16 {
+            self.0 as u16 - explicit_track_count
+        } else {
+            0
+        }
     }
 }