Fix lines in scope widget appearing under wrong group (#889)

When dap.ui.Layer.render is called with start == nil and end_ == nil, assign
start = 0 only when the buffer contains a single, empty line. For all other
cases, assign start == line-count instead of start == line-count - 1, which
appears to be causing the line swapping issue.

Add a simple test case where the buffer already contains non-empty lines, and a
new line is added with start == nil and end_ == nil, which should now be
appended to the end.

Author: ktakiman

lua/dap/ui.lua

@@ -466,12 +466,13 @@ function M.layer(buf)
       local modifiable = api.nvim_buf_get_option(buf, 'modifiable')
       api.nvim_buf_set_option(buf, 'modifiable', true)
       if not start and not end_ then
-        start = api.nvim_buf_line_count(buf) - 1
+        start = api.nvim_buf_line_count(buf)
         -- Avoid inserting a new line at the end of the buffer
         -- The case of no lines and one empty line are ambiguous;
         -- set_lines(buf, 0, 0) would "preserve" the "empty buffer line" while set_lines(buf, 0, -1) replaces it
         -- Need to use regular end_ = start in other cases to support injecting lines in all other cases
-        if start == 0 and (api.nvim_buf_get_lines(buf, 0, -1, true))[1] == "" then
+        if start == 1 and (api.nvim_buf_get_lines(buf, 0, -1, true))[1] == "" then
+          start = 0
           end_ = -1
         else
           end_ = start

tests/ui_spec.lua

@@ -62,6 +62,21 @@ describe('ui', function()
       assert.are.same('bbbb', layer.get(2).item.label)
       assert.are.same('', layer.get(3).item.label)
     end)
+
+    it('can append at the end', function()
+      layer.render({{ label = "e" }}, render_item, nil, nil, nil)
+      local lines = api.nvim_buf_get_lines(buf, 0, -1, true)
+      assert.are.same({
+        'aa',
+        'bbb',
+        'bbbb',
+        '',
+        'dd',
+        'e',
+      }, lines)
+      assert.are.same('dd', layer.get(4).item.label)
+      assert.are.same('e', layer.get(5).item.label)
+    end)
   end)
 
   local opts = {