Bug 952893. Part 3: Tighten TimeVarying invariants so that GetAt always returns an end time when the value actually changes. r=padenot

content/media/TimeVarying.h

@@ -44,6 +44,9 @@ protected:
  * The implementation records a mCurrent (the value at the current time)
  * and an array of "change times" (greater than the current time) and the
  * new value for each change time. This is a simple but dumb implementation.
+ * We maintain the invariant that each change entry in the array must have
+ * a different value to the value in the previous change entry (or, for
+ * the first change entry, mCurrent).
  */
 template <typename Time, typename T, uint32_t ReservedChanges>
 class TimeVarying : public TimeVaryingBase {
@@ -79,6 +82,9 @@ public:
       }
       mChanges.RemoveElementAt(i);
     }
+    if (mCurrent == aValue) {
+      return;
+    }
     mChanges.InsertElementAt(0, Entry(aTime, aValue));
   }
   /**