Support selective highlighting of lines

Instead of highlighting all lines when not all of them
are needed, only highlight from the beginning up to
the specified limit.

The `BlobPresenter#highlight` method has been updated
to support `to` param. This param will be used to limit
the content to be highlighted.

app/presenters/blob_presenter.rb

@@ -3,12 +3,12 @@
 class BlobPresenter < Gitlab::View::Presenter::Delegated
   presents :blob
 
-  def highlight(plain: nil)
+  def highlight(to: nil, plain: nil)
     load_all_blob_data
 
     Gitlab::Highlight.highlight(
       blob.path,
-      blob.data,
+      limited_blob_data(to: to),
       language: blob.language_from_gitattributes,
       plain: plain
     )
@@ -23,4 +23,18 @@ class BlobPresenter < Gitlab::View::Presenter::Delegated
   def load_all_blob_data
     blob.load_all_data! if blob.respond_to?(:load_all_data!)
   end
+
+  def limited_blob_data(to: nil)
+    return blob.data if to.blank?
+
+    # Even though we don't need all the lines at the start of the file (e.g
+    # viewing the middle part of a file), they still need to be highlighted
+    # to ensure that the succeeding lines can be formatted correctly (e.g.
+    # multi-line comments).
+    all_lines[0..to - 1].join
+  end
+
+  def all_lines
+    @all_lines ||= blob.data.lines
+  end
 end

app/presenters/blobs/unfold_presenter.rb

@@ -26,8 +26,6 @@ module Blobs
       # so we can accurately show the rest of the diff when unfolding.
       load_all_blob_data
 
-      @all_lines = blob.data.lines
-
       handle_full_or_end!
     end
 
@@ -46,7 +44,7 @@ module Blobs
 
     def lines
       strong_memoize(:lines) do
-        limit(highlight.lines).map(&:html_safe)
+        limit(highlight(to: to).lines).map(&:html_safe)
       end
     end
 
@@ -76,7 +74,7 @@ module Blobs
 
     def all_lines_size
       strong_memoize(:all_lines_size) do
-        @all_lines.size
+        all_lines.size
       end
     end
 
@@ -101,7 +99,7 @@ module Blobs
 
     def limited_blob_lines
       strong_memoize(:limited_blob_lines) do
-        limit(@all_lines)
+        limit(all_lines)
       end
     end
 

changelogs/unreleased/65152-unfolded-lines-perf-improvement.yml

+---
+title: Support selective highlighting of lines
+merge_request: 32514
+author:
+type: performance

spec/presenters/blob_presenter_spec.rb

@@ -39,6 +39,20 @@ describe BlobPresenter, :seed_helper do
       subject.highlight(plain: true)
     end
 
+    context '"to" param is present' do
+      before do
+        allow(git_blob)
+          .to receive(:data)
+          .and_return("line one\nline two\nline 3")
+      end
+
+      it 'returns limited highlighted content' do
+        expect(Gitlab::Highlight).to receive(:highlight).with('files/ruby/regex.rb', "line one\n", plain: nil, language: nil)
+
+        subject.highlight(to: 1)
+      end
+    end
+
     context 'gitlab-language contains a match' do
       before do
         allow(blob).to receive(:language_from_gitattributes).and_return('ruby')