Fix PlacementAmongFloats to avoid missing some bands (#30280)

PlacementAmongFloats would stop iterating when current_bands would be
empty, even if next_band wasn't at infinity.

Then the BFC root or replaced block was placed after all the floats,
even if it could fit next to some of them.

This patch moves the next_band into current_bands so that the loop
keeps considering bands.

components/layout_2020/flow/float.rs

@@ -200,14 +200,29 @@ impl<'a> PlacementAmongFloats<'a> {
         })
     }
 
+    /// Checks if we either have bands or we have gone past all of them.
+    /// This is an invariant that should hold, otherwise we are in a broken state.
+    fn has_bands_or_at_end(&self) -> bool {
+        !self.current_bands.is_empty() || self.next_band.top.px().is_infinite()
+    }
+
+    fn pop_front_band_ensuring_has_bands_or_at_end(&mut self) {
+        self.current_bands.pop_front();
+        if !self.has_bands_or_at_end() {
+            self.add_one_band();
+        }
+    }
+
     /// Run the placement algorithm for this [PlacementAmongFloats].
     pub(crate) fn place(&mut self) -> Rect<Length> {
+        debug_assert!(self.has_bands_or_at_end());
         while !self.current_bands.is_empty() {
             if let Some(result) = self.try_place_once() {
                 return result;
             }
-            self.current_bands.pop_front();
+            self.pop_front_band_ensuring_has_bands_or_at_end();
         }
+        debug_assert!(self.has_bands_or_at_end());
 
         // We could not fit the object in among the floats, so we place it as if it
         // cleared all floats.
@@ -237,6 +252,7 @@ impl<'a> PlacementAmongFloats<'a> {
         block_size_after_layout: Length,
         size_from_placement: &Vec2<Length>,
     ) -> bool {
+        debug_assert!(self.has_bands_or_at_end());
         debug_assert_eq!(size_from_placement.block, self.current_bands_height());
         debug_assert_eq!(
             size_from_placement.inline,
@@ -268,7 +284,7 @@ impl<'a> PlacementAmongFloats<'a> {
                 if available_inline_size < self.object_size.inline {
                     self.next_band = self.current_bands[old_num_bands];
                     self.current_bands.truncate(old_num_bands);
-                    self.current_bands.pop_front();
+                    self.pop_front_band_ensuring_has_bands_or_at_end();
                 }
                 return false;
             }