Documents: a press in a table opens its cell alone

Visual mode opened a pressed table whole, as Markdown source. A press
in a table now opens only that cell, in place, as its source in a
one-line field; the rest of the table stays drawn. Tab and Shift-Tab
walk the cells, Up and Down move a row or leave the table, Enter and
Escape write back in one undo step, and a bare pipe typed in a cell is
escaped on write-back. The table drawing moves to render/table.rs and
presses on a cell's padding name the cell.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/documents/document/tests/cells.rs

@@ -0,0 +1,58 @@
+//! A press in a table opens only its cell (R6.1).
+
+use gpui::{Modifiers, MouseButton, TestAppContext, VisualTestContext, point, px};
+
+use super::{open, opened, text};
+use crate::documents::MarkdownMode;
+
+const TABLE: &str = "| a | **bold** |\n| --- | --- |\n| c |  |\n";
+
+/// Presses cell `row`, `column` of the first table near its left.
+fn press_cell(row: usize, column: usize, cx: &mut VisualTestContext) {
+    let name = format!("markdown-cell-0-{row}-{column}");
+    let bounds = cx
+        .debug_bounds(Box::leak(name.into_boxed_str()))
+        .expect("the cell is drawn");
+    let at = point(bounds.left() + px(2.0), bounds.top() + px(2.0));
+    cx.simulate_mouse_down(at, MouseButton::Left, Modifiers::none());
+    cx.simulate_mouse_up(at, MouseButton::Left, Modifiers::none());
+    cx.run_until_parked();
+}
+
+#[gpui::test]
+fn a_press_opens_its_cell_and_writes_one_undo_step(cx: &mut TestAppContext) {
+    let (document, cx) = open(TABLE, MarkdownMode::Visual, cx);
+    press_cell(0, 1, cx);
+    assert_eq!(opened(&document, cx), "**bold**");
+    assert!(
+        cx.debug_bounds("markdown-cell-0-0-0").is_some(),
+        "the rest stays drawn"
+    );
+    press_cell(1, 1, cx);
+    assert_eq!(opened(&document, cx), "");
+    cx.simulate_input("x|y");
+    assert_eq!(
+        text(&document, cx),
+        "| a | **bold** |\n| --- | --- |\n| c | x\\|y |\n"
+    );
+    cx.simulate_keystrokes("escape");
+    let editor = document.read_with(cx, |document, _| document.editor().clone());
+    editor.update(cx, |editor, cx| editor.undo(cx));
+    assert_eq!(text(&document, cx), TABLE);
+}
+
+#[gpui::test]
+fn tab_and_arrows_walk_the_cells(cx: &mut TestAppContext) {
+    let (document, cx) = open(TABLE, MarkdownMode::Visual, cx);
+    press_cell(0, 0, cx);
+    assert_eq!(opened(&document, cx), "a");
+    cx.simulate_keystrokes("tab");
+    assert_eq!(opened(&document, cx), "**bold**");
+    cx.simulate_keystrokes("shift-tab");
+    assert_eq!(opened(&document, cx), "a");
+    cx.simulate_keystrokes("down");
+    assert_eq!(opened(&document, cx), "c");
+    cx.simulate_keystrokes("enter");
+    assert!(document.read_with(cx, |document, _| document.open.is_none()));
+    assert_eq!(text(&document, cx), TABLE);
+}