diff --git a/ports/geckolib/gecko_bindings/bindings.rs b/ports/geckolib/gecko_bindings/bindings.rs
index f983c039acc..49365492493 100644
--- a/ports/geckolib/gecko_bindings/bindings.rs
+++ b/ports/geckolib/gecko_bindings/bindings.rs
@@ -243,6 +243,8 @@ extern "C" {
     pub fn Gecko_GetNodeFlags(node: *mut RawGeckoNode) -> u32;
     pub fn Gecko_SetNodeFlags(node: *mut RawGeckoNode, flags: u32);
     pub fn Gecko_UnsetNodeFlags(node: *mut RawGeckoNode, flags: u32);
+    pub fn Gecko_ArrayEnsureCapacity(array: *mut ::std::os::raw::c_void,
+                                     capacity: usize, size: usize);
     pub fn Servo_StylesheetFromUTF8Bytes(bytes: *const u8, length: u32,
                                          parsing_mode: SheetParsingMode,
                                          base: *mut ThreadSafeURIHolder,
diff --git a/ports/geckolib/gecko_bindings/sugar/ns_style_auto_array.rs b/ports/geckolib/gecko_bindings/sugar/ns_style_auto_array.rs
index bfcd7dc3cef..78636b8fe14 100644
--- a/ports/geckolib/gecko_bindings/sugar/ns_style_auto_array.rs
+++ b/ports/geckolib/gecko_bindings/sugar/ns_style_auto_array.rs
@@ -17,5 +17,5 @@ impl<'a, T> IntoIterator for &'a mut nsStyleAutoArray<T> {
     type IntoIter = Chain<Once<&'a mut T>, IterMut<'a, T>>;
     fn into_iter(self) -> Self::IntoIter {
         self.iter_mut()
-    } 
+    }
 }
\ No newline at end of file
diff --git a/ports/geckolib/gecko_bindings/sugar/ns_t_array.rs b/ports/geckolib/gecko_bindings/sugar/ns_t_array.rs
index 57b6495b040..d24960b727f 100644
--- a/ports/geckolib/gecko_bindings/sugar/ns_t_array.rs
+++ b/ports/geckolib/gecko_bindings/sugar/ns_t_array.rs
@@ -2,8 +2,10 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+use bindings::Gecko_ArrayEnsureCapacity;
 use std::mem;
 use std::ops::{Deref, DerefMut};
+use std::os::raw::c_void;
 use std::slice;
 use structs::{nsTArray, nsTArrayHeader};
 
@@ -33,10 +35,27 @@ impl<T> nsTArray<T> {
         debug_assert!(!self.mBuffer.is_null());
         unsafe { mem::transmute(self.mBuffer) }
     }
+    fn header_mut <'a>(&'a mut self) -> &'a mut nsTArrayHeader {
+        debug_assert!(!self.mBuffer.is_null());
+        unsafe { mem::transmute(self.mBuffer) }
+    }
 
     #[inline]
     unsafe fn slice_begin(&self) -> *mut T {
         debug_assert!(!self.mBuffer.is_null());
         (self.mBuffer as *const nsTArrayHeader).offset(1) as *mut _
     }
+
+    fn ensure_capacity(&mut self, cap: usize) {
+        unsafe {
+            Gecko_ArrayEnsureCapacity(self as *mut nsTArray<T> as *mut c_void, cap, mem::size_of::<T>())
+        }
+    }
+
+    // unsafe because the array may contain uninits
+    pub unsafe fn set_len(&mut self, len: u32) {
+        self.ensure_capacity(len as usize);
+        let mut header = self.header_mut();
+        header.mLength = len;
+    }
 }