Context object passed to a *cngxPopoverArrow template.
The consumer's glyph receives the resolved panel edge (top / bottom /
left / right) and the inline-axis offset in CSS pixels that the
library would have placed the default diamond at. Either can be used to
rotate, translate, or theme the custom glyph so it tracks the trigger.
Inline-axis offset from the panel edge to the trigger's centre, in
CSS pixels. null before the first geometry read (the consumer
should treat null as "centre the glyph" - the default diamond
falls back to 50% in the same case).