The initial version of FX_TAB_SWITCH_COMPOSITE_E10S_MS skips recording the value for the case where we make the tab visible before the Layers have been created. This is because the parent process won't get notification for the composite triggered when the Layers are created, and thus we can't get an accurate result. The solution here is probably for the content process to listen for MozAfterPaint, since in this case the tab being displayed is triggered the child (by sending the layers), and that's where the notification will go. We'd then need to send the result timestamp back to the parent and record the result.