fix(snippet): setting end_right_gravity (#35061)

When right_gravity is set to true for deactivating tabstop expansion we
have to set end_right_gravity to false to avoid expanding the tabstop
region on the right side. Vice versa for activating tabstop expansion
again.

runtime/lua/vim/snippet.lua

@@ -119,7 +119,7 @@ local Tabstop = {}
 function Tabstop.new(index, bufnr, range, choices)
   local extmark_id = vim.api.nvim_buf_set_extmark(bufnr, snippet_ns, range[1], range[2], {
     right_gravity = true,
-    end_right_gravity = true,
+    end_right_gravity = false,
     end_line = range[3],
     end_col = range[4],
     hl_group = hl_group,
@@ -170,7 +170,7 @@ function Tabstop:set_right_gravity(right_gravity)
   local range = self:get_range()
   self.extmark_id = vim.api.nvim_buf_set_extmark(self.bufnr, snippet_ns, range[1], range[2], {
     right_gravity = right_gravity,
-    end_right_gravity = true,
+    end_right_gravity = not right_gravity,
     end_line = range[3],
     end_col = range[4],
     hl_group = hl_group,

test/functional/lua/snippet_spec.lua

@@ -249,7 +249,11 @@ describe('vim.snippet', function()
     feed('<Tab>')
     poke_eventloop()
     feed(',2')
-    eq({ 'for i=1,10,2 do', '\t', 'end' }, buf_lines(0))
+    -- Make sure changes on previous tabstops does not change following ones
+    feed('<S-Tab>')
+    poke_eventloop()
+    feed('20')
+    eq({ 'for i=1,20,2 do', '\t', 'end' }, buf_lines(0))
   end)
 
   it('updates snippet state when built-in completion menu is visible', function()