Let's say Alice, human magus 3, has just scribed an arcane scroll of shocking grasp using her Scribe Scroll feat. Alice suddenly feels malice toward her assistant, Bob the goblin.
Given the basic mechanism of Spell Combat:
As a full-round action, [a magus] can make all of his attacks with his melee weapon at a –2 penalty and can also cast any spell from the magus spell list with a casting time of 1 standard action.
Plus the following lines in the Using Items section of the Magic Items chapter (Core Rulebook 458):
Spell completion items are treated like spells in combat and do provoke attacks of opportunity.
Spell Completion: This is the activation method for scrolls. A scroll is a spell that is mostly finished. The preparation is done for the caster, so no preparation time is needed beforehand as with normal spellcasting. All that’s left to do is perform the finishing parts of the spellcasting (the final gestures, words, and so on). … Activating a spell completion item is a standard action (or the spell’s casting time, whichever is longer) and provokes attacks of opportunity exactly as casting a spell does.
And the following line in the Activate Magic Item section of Actions In Combat (Core Rulebook 184):
Spell Completion Items: Activating a spell completion item is the equivalent of casting a spell. It requires concentration and provokes attacks of opportunity. You lose the spell if your concentration is broken, and you can attempt to activate the item while on the defensive, as with casting a spell.
Can Alice punch Bob in the face, then cast the shocking grasp spell she just scribed as part of spell combat?
Best Answer
Let's begin assuming you are casting off of a "scroll" in the form of a spell-scar. From the SRD:
In this instance the "scroll" should be able to be used with spell-combat as it leaves your hand free for spell combat and it functions as casting a spell (see the sources cited in the question). I would note that while activating a scroll is a standard action, spell combat works with standard action spells (that being the whole point). Further from an RAI perspective it would be odd for this arcana (being a class feature of the magus) to not function with the primary feature of magi (Spell Combat).
Now let's assume that you are using a more traditional scroll and you can draw it as a swift action via some method (spring-loaded sheaths, being a tiefling, etc) or already have it drawn (because you theoretically could I suppose). Your hand is not precisely "free" but I would argue you can use spell combat anyway because of the existence of the Wand Wielder arcana which reads:
Two items of note:
tl;dr You should be able to cast from a scroll using spell combat assuming you didn't need to use a move action to draw it and that the spell is on your magus spell list.