Rename abs_x/abs_y to cumulative_x/cumulative_y

src/compute/taffy_tree.rs

@@ -278,28 +278,32 @@ fn perform_taffy_tree_hidden_layout(tree: &mut Taffy, node: NodeId) {
 }
 
 /// Rounds the calculated [`Layout`] to exact pixel values
+///
 /// In order to ensure that no gaps in the layout are introduced we:
-///   - Always round based on the absolute coordinates rather than parent-relative coordinates
+///   - Always round based on the cumulative x/y coordinates (relative to the viewport) rather than
+///     parent-relative coordinates
 ///   - Compute width/height by first rounding the top/bottom/left/right and then computing the difference
 ///     rather than rounding the width/height directly
-///
 /// See <https://github.com/facebook/yoga/commit/aa5b296ac78f7a22e1aeaf4891243c6bb76488e2> for more context
-fn round_layout(tree: &mut Taffy, node_id: NodeId, abs_x: f32, abs_y: f32) {
+///
+/// In order to prevent innacuracies caused by rounding already-rounded values, we read from `unrounded_layout`
+/// and write to `final_layout`.
+fn round_layout(tree: &mut Taffy, node_id: NodeId, cumulative_x: f32, cumulative_y: f32) {
     let node = &mut tree.nodes[node_id.into()];
     let unrounded_layout = node.unrounded_layout;
     let layout = &mut node.final_layout;
 
-    let abs_x = abs_x + unrounded_layout.location.x;
-    let abs_y = abs_y + unrounded_layout.location.y;
+    let cumulative_x = cumulative_x + unrounded_layout.location.x;
+    let cumulative_y = cumulative_y + unrounded_layout.location.y;
 
     layout.location.x = round(unrounded_layout.location.x);
     layout.location.y = round(unrounded_layout.location.y);
-    layout.size.width = round(abs_x + unrounded_layout.size.width) - round(abs_x);
-    layout.size.height = round(abs_y + unrounded_layout.size.height) - round(abs_y);
+    layout.size.width = round(cumulative_x + unrounded_layout.size.width) - round(cumulative_x);
+    layout.size.height = round(cumulative_y + unrounded_layout.size.height) - round(cumulative_y);
 
     let child_count = tree.child_count(node_id).unwrap();
     for index in 0..child_count {
         let child = tree.child(node_id, index);
-        round_layout(tree, child, abs_x, abs_y);
+        round_layout(tree, child, cumulative_x, cumulative_y);
     }
 }