Pierre-Eric, in my interpretation of OpenGL(R) ES Version 3.0.5 (November 3,
2016) Specification, Appendix 4: Shared Objects and Multiple Contexts, D.3.
Propagating Changes to Objects, Rule 4:...

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If the contents of an object T are changed in a context other than the current
context, T must be attached or re-attached to at least one binding point in the
current context, or at least one attachment point of a currently bound
container object C, in order to guarantee that the new contents of T are
visible in the current context.

Example: If a texture image is bound to multiple texture bind points and the
texture is changed in another context, re-binding the texture at any one of the
texture bind points is sufficient to cause the changes to be visible at all
texture bind points.
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...Finish or Fence are not required, since I am re-binding the texture on the
secondary thread where I expect changes to be seen.