feat: add overlap option to reduce virtual padding for overlapping images

When an image overlaps existing buffer lines (e.g. a rendered diagram
overlapping its code block), the new `overlap` option specifies how many
buffer lines the image covers. Virtual padding is reduced accordingly so
content below isn't pushed down unnecessarily.

Also simplifies render_offset_top handling to support negative values,
allowing images to be shifted upward relative to their anchor position.

Author: charlesg3

lua/image/image.lua

@@ -108,7 +108,8 @@ function Image:render(geometry)
 
     -- create extmark
     if was_rendered then
-      local total_height = height + (self.render_offset_top or 0)
+      -- subtract overlap lines to allow the rendered image to overlap with them
+      local total_height = math.max(0, height + (self.render_offset_top or 0) - (self.overlap or 0) + 1)
       local has_up_to_date_extmark = previous_extmark and previous_extmark.height == total_height
 
       if not has_up_to_date_extmark then
@@ -121,8 +122,7 @@ function Image:render(geometry)
         local filler = {}
         local extmark_opts = { id = self.internal_id, strict = false }
         if self.with_virtual_padding then
-          -- only reserve real height for the extmark, padding is applied during rendering
-          local total_lines = height
+          local total_lines = math.max(0, height + (self.render_offset_top or 0) - (self.overlap or 0) + 1)
           for _ = 0, total_lines - 1 do
             filler[#filler + 1] = { { " ", "" } }
           end
@@ -298,6 +298,7 @@ local from_file = function(path, options, state)
         inline = opts.inline or opts.with_virtual_padding or false,
         is_rendered = false,
         render_offset_top = opts.render_offset_top or 0,
+        overlap = opts.overlap or 0,
         crop_hash = nil,
         resize_hash = nil,
         namespace = opts.namespace or nil,

lua/image/renderer.lua

@@ -194,11 +194,7 @@ local render = function(image)
   if image.window == nil then
     absolute_x = original_x
     absolute_y = original_y
-    -- apply render_offset_top
-    if image.render_offset_top and image.render_offset_top > 0 then
-      --
-      absolute_y = absolute_y + image.render_offset_top
-    end
+    absolute_y = absolute_y + (image.render_offset_top or 0)
   else
     -- get window object
     local window = nil
@@ -317,10 +313,10 @@ local render = function(image)
       absolute_x = screen_pos.col - 1
       absolute_y = screen_pos.row
     end
-    -- apply render_offset_top offset if set (but not for floating windows and not during partial scroll)
+    -- apply render_offset_top except for floating windows or during partial scroll
     local is_floating = window and window.is_floating or false
-    if image.render_offset_top and image.render_offset_top > 0 and not is_floating and not is_partial_scroll then
-      absolute_y = absolute_y + image.render_offset_top
+    if not is_floating and not is_partial_scroll then
+      absolute_y = absolute_y + (image.render_offset_top or 0)
     end
   end