[Meo-Ada Mespotine]

Here's some code on how to make hovering working.
It's very basic, but you should get the idea...

-- Mouse hovering demo for gfx-windows in Reaper
--
-- Meo Mespotine, 9.9.2019
--
-- this draws some simplified MIDI-keyboard-keys into a window and shows additional graphics/images, if the mouse is
-- hovering above a key


-- prepare some variables
xoffs_keys=10       -- x-offset for the MIDI-keys
yoffs_keys=10       -- y-offset for the MIDI-keys
width_of_keys=100   -- width of the MIDI-keys
height_of_keys=30   -- height of the MIDI-keys; minimum is 4 in this demo

keys_image_buffer=1 -- the image-buffer for the MIDI-keys

add_image_buffer=2  -- the first image-buffer for the additional hover images(called add-image in this script)
                    -- must be bigger than keys_image_buffer in this demo!
add_image_x=200     -- the x-position of the add-image
add_image_y=500     -- the y-position of the add-image
add_loaded_images={}-- the image-indices of the loaded add-images; will be set by function LoadImages

function LoadImages()
  -- this loads up to 100 images we will display, when the mouse is hovering above a MIDI-key
  -- this loads them from the resource-path/Data/toolbar_icons, but you can choose anything else as well
  -- the table add_loaded_images holds the imagebuffer-indexes of all loaded files for your own reference
  
  imagebuffer=add_image_buffer
  for i=1, 100 do
    -- load only png-files
    ImageFilename=reaper.EnumerateFiles(reaper.GetResourcePath().."/Data/toolbar_icons", i)
    if ImageFilename==nil then break end
    if ImageFilename:match("%.png")~=nil then 
      add_loaded_images[imagebuffer]=gfx.loadimg(imagebuffer, reaper.GetResourcePath().."/Data/toolbar_icons/"..ImageFilename)
      imagebuffer=imagebuffer+1
    end
  end
end

function drawkeys()
  -- this draws some simple MIDI-keys
  -- not pretty, but enough for this demo
  local buf=gfx.dest
  gfx.dest=keys_image_buffer
  for i=yoffs_keys, 700, height_of_keys do
    gfx.rect(xoffs_keys, i, width_of_keys, height_of_keys-3)
  end
  gfx.dest=buf
end

function check_for_hover()
  -- let's check, whether the mouse is above any MIDI-key and return its index
  -- if not hovering above a key, return -1
  local X,Y=gfx.mouse_x, gfx.mouse_y
  local key=0
  for i=yoffs_keys, 700, height_of_keys do
    -- this iterates through all existing keys and checks, whether a key is currently hovered over or not
    key=key+1
    if Y>=i and Y<=i+(height_of_keys-3)                    -- check if mouse if hovering over a key -> Y-position
        and X>=xoffs_keys and X<=xoffs_keys+width_of_keys  -- check if mouse if hovering over a key -> X-position
    then
       -- draw a rectangle above the found key
       gfx.set(1,0,0,1)
       gfx.rect(xoffs_keys+7, i+6, width_of_keys-14, height_of_keys-14, 1)

       -- return the MIDI-keyindex
       return key 
    end
  end
  
  return -1
end

function add_blit_image(imagefileindex)
  -- if the imagefileindex is greater/equal 1, this blits the accompanying image
  -- if not, it displays it not at all
  if imagefileindex<1 then return end
  
  -- get current x and y-coordinates
  local tempx,tempy=gfx.x, gfx.y    
  
  -- set drawing-coordinates for add-image               
  gfx.x=add_image_x
  gfx.y=add_image_y
  
  -- blit the add-image into framebuffer
  gfx.blit(add_image_buffer+imagefileindex, 1, 0)
  
  -- restore old drawing-coordinates
  gfx.x=tempx
  gfx.y=tempy
end

function main()
  gfx.blit(keys_image_buffer,1,0) -- put the MIDI-keys into the framebuffer
  keyindex=check_for_hover()      -- get the currently hovered key
  add_blit_image(keyindex)        -- put the add-image associated with a specific key into the framebuffer
  reaper.defer(main)              -- repeat it all over again
end


-- now let's begin the show
gfx.init("MIDI", 500,800,0,1,1)               -- open the window
gfx.setimgdim(keys_image_buffer, 2048, 2048)  -- set dimensions for framebuffer for the MIDI-keys
LoadImages()                                  -- load some default-images from Reaper we display, if the mouse is hovering above a MIDI-key
drawkeys()                                    -- let's draw the MIDI-keys

main()                                        -- start the program