Fix floating point errors in table layout (#33098)

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

components/layout_2020/table/layout.rs

@@ -890,17 +890,45 @@ impl<'a> TableLayout<'a> {
         }
 
         let bounds = |sum_a, sum_b| self.assignable_width > sum_a && self.assignable_width < sum_b;
+
         let blend = |a: &[Au], sum_a: Au, b: &[Au], sum_b: Au| {
             // First convert the Au units to f32 in order to do floating point division.
             let weight_a =
                 (self.assignable_width - sum_b).to_f32_px() / (sum_a - sum_b).to_f32_px();
             let weight_b = 1.0 - weight_a;
-            a.iter()
+
+            let mut sum_accounting_for_floating_point_inaccuracy = Au::new(0);
+            let mut widths: Vec<Au> = a
+                .iter()
                 .zip(b.iter())
                 .map(|(guess_a, guess_b)| {
                     (guess_a.scale_by(weight_a)) + (guess_b.scale_by(weight_b))
                 })
-                .collect()
+                .inspect(|&column_width| {
+                    sum_accounting_for_floating_point_inaccuracy += column_width;
+                })
+                .collect();
+
+            if sum_accounting_for_floating_point_inaccuracy != self.assignable_width {
+                // The computations above can introduce floating-point imprecisions.
+                // Since these errors are very small (+-1Au), it's fine to simply adjust
+                // the first column such that the total width matches the assignable width
+                let difference =
+                    self.assignable_width - sum_accounting_for_floating_point_inaccuracy;
+
+                debug_assert!(
+                    difference.abs() <= Au::new(widths.len() as i32),
+                    "A deviation of more than one Au per column is unlikely to be caused by float imprecision"
+                );
+
+                // We checked if the table was empty at the top of the function, so there
+                // always is a first column
+                widths[0] += difference;
+            }
+
+            debug_assert!(widths.iter().sum::<Au>() == self.assignable_width);
+
+            widths
         };
 
         if bounds(min_content_sizes_sum, min_content_percentage_sizing_sum) {