Authors CurseForge

Not sure how much this will help, but here's the relevant code from my scrolling dropdown widget library; shouldn't be too hard to adapt.

Creating:
https://github.com/Phanx/PhanxConfig-Dropdown/blob/master/PhanxConfig-Dropdown.lua#L271

Updating:
https://github.com/Phanx/PhanxConfig-Dropdown/blob/master/PhanxConfig-Dropdown.lua#L197

Then in your code, instead of doing this:

for SpellID, Name in pairs(SpellList) do
   if strfind(Name, SearchSt) then
      if ActiveButtons < PredictorLines then
         -- fill in the next button
         ActiveButtons = ActiveButtons + 1
      else
         break
      end
end

...do this first to fill in a list of matches:

local matches = {}
for SpellID, Name in pairs(SpellList) do
   if strfind(Name, SearchSt) then
      matches[#matches+1] = SpellID
   end
end

...and then update your buttons based on that + the scroll offset.

On a side note, you should use string.find instead of string.match in your search code -- when all you need to know is "does this string contain this other string?" it's almost twice as fast to find instead of match.

Also, your current code will show the matches in undefined order, so you may want to sort the matches by spell name before you update your buttons:

-- define once:
local function sortMatches(a, b)
   return SpellList[a] < SpellList[b]
end
-- call later in search routine:
table.sort(matches, sortMatches)

You could also take advantage of the fact that string.find tells you where in the string the search term was found, and prioritize spells that start with the search term over spells that just contain the search term:

local matches, prioritize = {}, {}
for SpellID, Name in pairs(SpellList) do
   local start = strfind(Name, SearchSt)
   if start then
      matches[#matches+1] = SpellID
      prioritize[SpellID] = start == 1
   end
end

local function sortMatches(a, b)
   if prioritize[a] == prioritize[b] then
      return SpellList[a] < SpellList[b]
   else
      return prioritize[a] and a or b
   end
end