Normative: When TA.copyWithin argument handling shrinks TA, use whatever elements still fit (tc39#3619)

...regardless of copy direction

Fixes tc39#3618

This aligns the spec with implementation reality.

Author: gibson042

spec.html

@@ -41732,9 +41732,10 @@ <h1>%TypedArray%.prototype.copyWithin ( _target_, _start_ [ , _end_ ] )</h1>
             1. Set _taRecord_ to MakeTypedArrayWithBufferWitnessRecord(_O_, ~seq-cst~).
             1. If IsTypedArrayOutOfBounds(_taRecord_) is *true*, throw a *TypeError* exception.
             1. Set _len_ to TypedArrayLength(_taRecord_).
+            1. NOTE: Side-effects of the above steps may have reduced the size of _O_, in which case copying should proceed with the longest still-applicable prefix.
+            1. Set _count_ to min(_count_, _len_ - _startIndex_, _len_ - _targetIndex_).
             1. Let _elementSize_ be TypedArrayElementSize(_O_).
             1. Let _byteOffset_ be _O_.[[ByteOffset]].
-            1. Let _bufferByteLimit_ be (_len_ × _elementSize_) + _byteOffset_.
             1. Let _toByteIndex_ be (_targetIndex_ × _elementSize_) + _byteOffset_.
             1. Let _fromByteIndex_ be (_startIndex_ × _elementSize_) + _byteOffset_.
             1. Let _countBytes_ be _count_ × _elementSize_.
@@ -41745,14 +41746,11 @@ <h1>%TypedArray%.prototype.copyWithin ( _target_, _start_ [ , _end_ ] )</h1>
             1. Else,
               1. Let _direction_ be 1.
             1. Repeat, while _countBytes_ > 0,
-              1. If _fromByteIndex_ &lt; _bufferByteLimit_ and _toByteIndex_ &lt; _bufferByteLimit_, then
-                1. Let _value_ be GetValueFromBuffer(_buffer_, _fromByteIndex_, ~uint8~, *true*, ~unordered~).
-                1. Perform SetValueInBuffer(_buffer_, _toByteIndex_, ~uint8~, _value_, *true*, ~unordered~).
-                1. Set _fromByteIndex_ to _fromByteIndex_ + _direction_.
-                1. Set _toByteIndex_ to _toByteIndex_ + _direction_.
-                1. Set _countBytes_ to _countBytes_ - 1.
-              1. Else,
-                1. Set _countBytes_ to 0.
+              1. Let _value_ be GetValueFromBuffer(_buffer_, _fromByteIndex_, ~uint8~, *true*, ~unordered~).
+              1. Perform SetValueInBuffer(_buffer_, _toByteIndex_, ~uint8~, _value_, *true*, ~unordered~).
+              1. Set _fromByteIndex_ to _fromByteIndex_ + _direction_.
+              1. Set _toByteIndex_ to _toByteIndex_ + _direction_.
+              1. Set _countBytes_ to _countBytes_ - 1.
           1. Return _O_.
         </emu-alg>
       </emu-clause>