There are some passive mana generation sources that display on the bar where the player is not able to make actions while they are channeled, e.g. Symbol of Hope or Potion of Spiritual Clarity. For these, the bar factors in passive mana regen to the estimated passive total (and the threshold line for channeled potions). By default, these factor in base mana regen modifiers like Enlightenment (permanent passive talent) or Mana Tide Totem (temporary buff).

As MTT can expire before SoH or PoSC finishes, the estimated mana regen may not be accurate unless the bar also considers the time MTT is active and expires.

This will eventually matter for all healers, but only Holy Priest is implemented right now.

Low priority for the moment.

It is doubtful I'll ever get around to this.