GfxDevice::UpdateBufferRanges when running UIBuilder tests

Steps to reproduce:

1. Open UIBuilder tests
2. Run ViewportToolbarTests

Actual results: 

On some machines, we get an assert failure such as:

GfxDevice::UpdateBufferRanges: range out of bounds @ 0 of 3: offsetFromWriteStart=28288 + size=384 > mappedRangeSize=5376

Expected results: 

No assert should fail

Reproducible with versions: 6000.x

Not reproducible with versions: 

Can’t test with versions: 

Tested on (OS): 

Notes:

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