Fix editor hang when positioned above viewport (#45077)

Fixes the hang introduced in #44995 (which was reverted in #45011) and
re-enables the optimization.

## Background

PR #44995 introduced an optimization to skip rendering lines that are
clipped by parent containers (e.g., when a large AutoHeight editor is
inside a scrollable List). This significantly improved performance for
large diffs in the Agent Panel.
However, #45011 reverted this change because it caused the main thread
to hang for 100+ seconds in certain scenarios, requiring a force quit to
recover.

## Root Cause
The original analysis in #45011 suggested that visible_bounds wasn’t
being intersected properly, but that was incorrect—the intersection via
with_content_mask works correctly. The actual bug: when an editor is
positioned above the visible viewport (e.g., scrolled past in a List),
the clipping calculation produces a start_row that exceeds max_row:

1. Editor’s bounds.origin.y becomes very negative (e.g., -10000px)
2. After intersection, visible_bounds.origin.y is at the viewport top
(e.g., 0)
3. clipped_top_in_lines = (0 - (-10000)) / line_height = huge number
4. start_row = huge number, but end_row is clamped to max_row
5. This creates an invalid range where start_row > end_row

This caused two different failures depending on build mode:
- Debug mode: Panic from subtraction overflow in
Range<DisplayRow>::len()
- Release mode: Integer wraparound causing blocks_in_range to enter an
infinite loop (the 100+ second hang)

## Fix

Simply clamp start_row to max_row, ensuring the row range is always
valid:

```rs
let start_row = cmp::min(
    DisplayRow((scroll_position.y + clipped_top_in_lines).floor() as u32),
    max_row,
);
```

## Testing
Added a regression test that draws an editor at y=-10000 to simulate an
editor that’s been scrolled past in a List. This would panic in debug
mode (and hang in release mode) before the fix.

Release Notes:
- Improved agent panel performance when rendering large diffs.

crates/editor/src/editor_tests.rs

@@ -29530,3 +29530,38 @@ async fn test_local_worktree_trust(cx: &mut TestAppContext) {
         trusted_worktrees.update(cx, |store, cx| store.can_trust(worktree_id, cx));
     assert!(can_trust_after, "worktree should be trusted after trust()");
 }
+
+#[gpui::test]
+fn test_editor_rendering_when_positioned_above_viewport(cx: &mut TestAppContext) {
+    // This test reproduces a bug where drawing an editor at a position above the viewport
+    // (simulating what happens when an AutoHeight editor inside a List is scrolled past)
+    // causes an infinite loop in blocks_in_range.
+    //
+    // The issue: when the editor's bounds.origin.y is very negative (above the viewport),
+    // the content mask intersection produces visible_bounds with origin at the viewport top.
+    // This makes clipped_top_in_lines very large, causing start_row to exceed max_row.
+    // When blocks_in_range is called with start_row > max_row, the cursor seeks to the end
+    // but the while loop after seek never terminates because cursor.next() is a no-op at end.
+    init_test(cx, |_| {});
+
+    let window = cx.add_window(|_, _| gpui::Empty);
+    let mut cx = VisualTestContext::from_window(*window, cx);
+
+    let buffer = cx.update(|_, cx| MultiBuffer::build_simple("a\nb\nc\nd\ne\nf\ng\nh\ni\nj\n", cx));
+    let editor = cx.new_window_entity(|window, cx| build_editor(buffer, window, cx));
+
+    // Simulate a small viewport (500x500 pixels at origin 0,0)
+    cx.simulate_resize(gpui::size(px(500.), px(500.)));
+
+    // Draw the editor at a very negative Y position, simulating an editor that's been
+    // scrolled way above the visible viewport (like in a List that has scrolled past it).
+    // The editor is 3000px tall but positioned at y=-10000, so it's entirely above the viewport.
+    // This should NOT hang - it should just render nothing.
+    cx.draw(
+        gpui::point(px(0.), px(-10000.)),
+        gpui::size(px(500.), px(3000.)),
+        |_, _| editor.clone(),
+    );
+
+    // If we get here without hanging, the test passes
+}

crates/editor/src/element.rs

@@ -9164,6 +9164,15 @@ impl Element for EditorElement {
                     let height_in_lines = f64::from(bounds.size.height / line_height);
                     let max_row = snapshot.max_point().row().as_f64();
 
+                    // Calculate how much of the editor is clipped by parent containers (e.g., List).
+                    // This allows us to only render lines that are actually visible, which is
+                    // critical for performance when large AutoHeight editors are inside Lists.
+                    let visible_bounds = window.content_mask().bounds;
+                    let clipped_top = (visible_bounds.origin.y - bounds.origin.y).max(px(0.));
+                    let clipped_top_in_lines = f64::from(clipped_top / line_height);
+                    let visible_height_in_lines =
+                        f64::from(visible_bounds.size.height / line_height);
+
                     // The max scroll position for the top of the window
                     let max_scroll_top = if matches!(
                         snapshot.mode,
@@ -9220,10 +9229,16 @@ impl Element for EditorElement {
                     let mut scroll_position = snapshot.scroll_position();
                     // The scroll position is a fractional point, the whole number of which represents
                     // the top of the window in terms of display rows.
-                    let start_row = DisplayRow(scroll_position.y as u32);
+                    // We add clipped_top_in_lines to skip rows that are clipped by parent containers,
+                    // but we don't modify scroll_position itself since the parent handles positioning.
                     let max_row = snapshot.max_point().row();
+                    let start_row = cmp::min(
+                        DisplayRow((scroll_position.y + clipped_top_in_lines).floor() as u32),
+                        max_row,
+                    );
                     let end_row = cmp::min(
-                        (scroll_position.y + height_in_lines).ceil() as u32,
+                        (scroll_position.y + clipped_top_in_lines + visible_height_in_lines).ceil()
+                            as u32,
                         max_row.next_row().0,
                     );
                     let end_row = DisplayRow(end_row);

crates/editor/src/test.rs

@@ -176,11 +176,9 @@ pub fn block_content_for_tests(
 }
 
 pub fn editor_content_with_blocks(editor: &Entity<Editor>, cx: &mut VisualTestContext) -> String {
-    cx.draw(
-        gpui::Point::default(),
-        size(px(3000.0), px(3000.0)),
-        |_, _| editor.clone(),
-    );
+    let draw_size = size(px(3000.0), px(3000.0));
+    cx.simulate_resize(draw_size);
+    cx.draw(gpui::Point::default(), draw_size, |_, _| editor.clone());
     let (snapshot, mut lines, blocks) = editor.update_in(cx, |editor, window, cx| {
         let snapshot = editor.snapshot(window, cx);
         let text = editor.display_text(cx);