fix bug: lines in buffer can only ever be { "" } at minimum

Author: jrop

lua/morph.lua

@@ -963,6 +963,10 @@ function Morph:render(tree)
     end,
   }
 
+  -- Edge case: empty trees produce empty lines array, but buffers always have
+  -- at least one line. Set curr.lines to reflect reality, not the empty tree.
+  if #lines == 0 then lines = { '' } end
+
   -- Update buffer text with minimal edits
   --- @diagnostic disable-next-line: assign-type-mismatch
   self.text_content.old = self.text_content.curr

spec/morph_spec.lua

@@ -3437,4 +3437,32 @@ describe('Morph', function()
       end)
     end)
   end)
+
+  ----------------------------------------------------------------------------
+  -- REGRESSION TEST: Rendering empty tree should not cause blank lines
+  --
+  -- Bug: When rendering an empty tree ({}) between non-empty renders,
+  -- the next render would have old.lines = {} (empty array) instead of
+  -- {""}, causing patch_lines to add an extra blank line.
+  ----------------------------------------------------------------------------
+
+  it('should not add blank lines when rendering empty tree between content', function()
+    with_buf({}, function()
+      local r = Morph.new(0)
+
+      -- First render: non-empty content
+      r:render { 'First' }
+      assert.are.same({ 'First' }, get_lines())
+
+      -- Second render: empty tree (may or may not clear buffer)
+      r:render {}
+
+      -- Third render: new non-empty content
+      -- The key assertion: no blank line should appear after the content
+      r:render { 'Second' }
+      local lines = get_lines()
+      assert.are.same(1, #lines, 'Buffer should have exactly 1 line, no blank lines')
+      assert.are.same('Second', lines[1])
+    end)
+  end)
 end)