Pieter, is not the first time that a member of your team give me dictatorial order in this forums and in others, like (Google+ communities).
The quality is not a rapid response. May be better a GOOD RESPONSE.
A good response starts with an empathic attitude.
Not thinking that the customer is wrong, but thinking, something like "What can be the problem of this customer, that make him dedicate time to redact a question here?".
Have you made an statistic about the questions I made, how many times I've accepted the response as good?
Do it, please!
The behavior you show in the video is by design. It is not a bug, it is the designed behavior.
I invite you a fifth time to read:
ColumnSizing = true -> sizing can be done from cell border of normal cells
FixedColumnSizing = true -> sizing can be done from cell border of fixed cells, the column header cells happen to be fixed cells, hence FixedColumnSizing = true let's you resize from the fixed column header cells
If we have a disagreement about the choice of a property name, then you will have to accept that disagreement because we are not going to change this property name as we do not want to break backwards compatibility.
If it fits in your mind better that this property would be
ColumnSizingByFixedColumnHeader: boolean
then do a find & replace in the source code of FixedColumnSizing by ColumnSizingByFixedColumnHeader
