Fix diff commenting results just after changing view

When you change the diff view (inline / side-by-side), we set a cookie based on
that new view. When you add a comment, we choose the style to use in the
response based on that cookie.

However, when you have just changed diff style, the request cookie will contain
the old value, so we should use the view param instead.

app/helpers/diff_helper.rb

@@ -15,7 +15,7 @@ module DiffHelper
   def diff_view
     @diff_view ||= begin
       diff_views = %w(inline parallel)
-      diff_view = cookies[:diff_view]
+      diff_view = params[:view] || cookies[:diff_view]
       diff_view = diff_views.first unless diff_views.include?(diff_view)
       diff_view.to_sym
     end

changelogs/unreleased/35695-comment-appears-in-a-wrong-place-after-changing-diff-view-to-inline.yml

+---
+title: Fix display of new diff comments after changing b between diff views
+merge_request:
+author:

spec/helpers/diff_helper_spec.rb

@@ -12,19 +12,32 @@ describe DiffHelper do
   let(:diff_file) { Gitlab::Diff::File.new(diff, diff_refs: diff_refs, repository: repository) }
   describe 'diff_view' do
+    it 'uses the view param over the cookie' do
+      controller.params[:view] = 'parallel'
+      helper.request.cookies[:diff_view] = 'inline'
+
+      expect(helper.diff_view).to eq :parallel
+    end
+
+    it 'returns the default value when the view param is invalid' do
+      controller.params[:view] = 'invalid'
+
+      expect(helper.diff_view).to eq :inline
+    end
+
     it 'returns a valid value when cookie is set' do
       helper.request.cookies[:diff_view] = 'parallel'
       expect(helper.diff_view).to eq :parallel
     end
-    it 'returns a default value when cookie is invalid' do
+    it 'returns the default value when cookie is invalid' do
       helper.request.cookies[:diff_view] = 'invalid'
       expect(helper.diff_view).to eq :inline
     end
-    it 'returns a default value when cookie is nil' do
+    it 'returns the default value when cookie is nil' do
       expect(helper.request.cookies).to be_empty
       expect(helper.diff_view).to eq :inline