get("#348") -- rgb functions
get("#388") -- rect functions
get("#410") -- shrinkRectangle
get("#380") -- hashImage

w, h = img.width, img.height
g = 3 -- grid size
maxBlackPixels = g-1
minSize = {20, 20} -- exclude the really small white spaces
gw, gh = math.floor(w/g), math.floor(h/g) -- width & height of grid

grid = {}
for gy = 0, h-g, g do
  for gx = 0, w-g, g do
    local black = 0
    for y = gy, gy+g-1 do
      for x = gx, gx+g-1 do
        local b = bright(rgb(img.getInt(x, y)))
        if b < 0.94 then
          black = black+1
          if black > maxBlackPixels then
            goto next
          end
        end
      end
    end
    grid[(gy/g)*gw+(gx/g)+1] = true
    ::next::
  end
end

-- stack layout: {{x, y}, ...}
local r
function fill(stack)
  while #stack > 0 do
    local x, y = unpack(stack[#stack])
    stack[#stack] = nil
    
    if not (x < 0 or y < 0 or x >= gw or y >= gh) then
      local idx = y*gw+x+1
      
      if grid[idx] then
        grid[idx] = nil
        local me = newRectangle(x, y, 1, 1)
        if r == nil then
          r = me
        else
          r = mergeRectangles(r, me)
        end
        
        stack[#stack+1] = {x-1, y}
        stack[#stack+1] = {x+1, y}
        stack[#stack+1] = {x, y-1}
        stack[#stack+1] = {x, y+1}
      end
    end
  end
end

result = {}
for y = 0, gh-1 do
  for x = 0, gw-1 do
    r = nil
    fill {{x, y, 1}}
    if r and r.width >= minSize[1] and r.height >= minSize[2] then
      --table.insert(result, (x*g).."/"..(y*g).."-"..c)
      r = scaleRectangle(r, g)
      --r = shrinkRectangle(r, img)
      table.insert(result, recttostring(r))
    end
  end
end

return "White Spaces: "..table.concat(result, "|")

IDEA uses FFFFFF for general background and FFFFD7 for cursor line -> bright = 0.9477
The borders have 9B9B9B which has brightness 0.607. However, it's only one pixel wide, so we should really check for ANY dark pixels and stop there.
The "gutter" is F5F5F5 which is 0.96.