editor: Fix horizontal scrollbar alignment if indent guides are disabled (#31621)

Finn Evers created 

Follow-up to #24887
Follow-up to #31510

This PR ensures that [this misalignment of the horizontal
scrollbar](https://github.com/zed-industries/zed/pull/31510#issuecomment-2912842457)
does not occur. See the entire discussion in the first linked PR as to
why this gap is there in the first place.

I am also aware of the general stance towards comments. Yet, I felt for
this case it is better to just straight up explain how these two things
are connected, as I do believe this is not intuitively clear after all.

Might also be a good time to bring
https://github.com/zed-industries/zed/issues/25519 up again. The
horizontal scrollbar seems huge for the edit file tool card.
Furthermore, since we do not reserve space for the horizontal scrollbar
(yet), this will lead to the last line being not clickable.

Release Notes:

- N/A

Change summary

crates/editor/src/element.rs | 11 +++++++++++
1 file changed, 11 insertions(+)

Detailed changes

crates/editor/src/element.rs 🔗 

@@ -1512,6 +1512,17 @@ impl EditorElement {
             ShowScrollbar::Never => return None,
         };
 
+        // The horizontal scrollbar is usually slightly offset to align nicely with
+        // indent guides. However, this offset is not needed if indent guides are
+        // disabled for the current editor.
+        let content_offset = self
+            .editor
+            .read(cx)
+            .show_indent_guides
+            .is_none_or(|should_show| should_show)
+            .then_some(content_offset)
+            .unwrap_or_default();
+
         Some(EditorScrollbars::from_scrollbar_axes(
             ScrollbarAxes {
                 horizontal: scrollbar_settings.axes.horizontal