indent guides: Fix issue with entirely-whitespace lines (#31916)

Closes #26957

Release Notes:

- Fix an edge case where indent guides would be rendered incorrectly if
lines consisted of entirely whitespace

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>

crates/editor/src/editor_tests.rs

@@ -17127,6 +17127,64 @@ async fn test_indent_guide_ends_before_empty_line(cx: &mut TestAppContext) {
     );
 }
 
+#[gpui::test]
+async fn test_indent_guide_ignored_only_whitespace_lines(cx: &mut TestAppContext) {
+    let (buffer_id, mut cx) = setup_indent_guides_editor(
+        &"
+        function component() {
+        \treturn (
+        \t\t\t
+        \t\t<div>
+        \t\t\t<abc></abc>
+        \t\t</div>
+        \t)
+        }"
+        .unindent(),
+        cx,
+    )
+    .await;
+
+    assert_indent_guides(
+        0..8,
+        vec![
+            indent_guide(buffer_id, 1, 6, 0),
+            indent_guide(buffer_id, 2, 5, 1),
+            indent_guide(buffer_id, 4, 4, 2),
+        ],
+        None,
+        &mut cx,
+    );
+}
+
+#[gpui::test]
+async fn test_indent_guide_fallback_to_next_non_entirely_whitespace_line(cx: &mut TestAppContext) {
+    let (buffer_id, mut cx) = setup_indent_guides_editor(
+        &"
+        function component() {
+        \treturn (
+        \t
+        \t\t<div>
+        \t\t\t<abc></abc>
+        \t\t</div>
+        \t)
+        }"
+        .unindent(),
+        cx,
+    )
+    .await;
+
+    assert_indent_guides(
+        0..8,
+        vec![
+            indent_guide(buffer_id, 1, 6, 0),
+            indent_guide(buffer_id, 2, 5, 1),
+            indent_guide(buffer_id, 4, 4, 2),
+        ],
+        None,
+        &mut cx,
+    );
+}
+
 #[gpui::test]
 async fn test_indent_guide_continuing_off_screen(cx: &mut TestAppContext) {
     let (buffer_id, mut cx) = setup_indent_guides_editor(