terminal: RenderState linkCells needs to use Page y not Viewport y

Fixes #9957

Our `Page.getRowAndCell` uses a _page-relative_ x/y coordinate system
and we were passing in viewport x/y. This has the possibility to leading
to all sorts of bugs, including the crash found in #9957 but also simply
reading the wrong cell even in single-page scenarios.

src/terminal/render.zig

@@ -817,11 +817,12 @@ pub const RenderState = struct {
         const row_cells = row_slice.items(.cells);
 
         // Grab our link ID
-        const link_page: *page.Page = &row_pins[viewport_point.y].node.data;
+        const link_pin: PageList.Pin = row_pins[viewport_point.y];
+        const link_page: *page.Page = &link_pin.node.data;
         const link = link: {
             const rac = link_page.getRowAndCell(
                 viewport_point.x,
-                viewport_point.y,
+                link_pin.y,
             );
 
             // The likely scenario is that our mouse isn't even over a link.
@@ -848,7 +849,7 @@ pub const RenderState = struct {
 
                 const other_page: *page.Page = &pin.node.data;
                 const other = link: {
-                    const rac = other_page.getRowAndCell(x, y);
+                    const rac = other_page.getRowAndCell(x, pin.y);
                     const link_id = other_page.lookupHyperlink(rac.cell) orelse continue;
                     break :link other_page.hyperlink_set.get(
                         other_page.memory,
@@ -1317,6 +1318,48 @@ test "string" {
     try testing.expectEqualStrings(expected, result);
 }
 
+test "linkCells with scrollback spanning pages" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    const viewport_rows: size.CellCountInt = 10;
+    const tail_rows: size.CellCountInt = 5;
+
+    var t = try Terminal.init(alloc, .{
+        .cols = page.std_capacity.cols,
+        .rows = viewport_rows,
+        .max_scrollback = 10_000,
+    });
+    defer t.deinit(alloc);
+
+    var s = t.vtStream();
+    defer s.deinit();
+
+    const pages = &t.screens.active.pages;
+    const first_page_cap = pages.pages.first.?.data.capacity.rows;
+
+    // Fill first page
+    for (0..first_page_cap - 1) |_| try s.nextSlice("\r\n");
+
+    // Create second page with hyperlink
+    try s.nextSlice("\r\n");
+    try s.nextSlice("\x1b]8;;http://example.com\x1b\\LINK\x1b]8;;\x1b\\");
+    for (0..(tail_rows - 1)) |_| try s.nextSlice("\r\n");
+
+    var state: RenderState = .empty;
+    defer state.deinit(alloc);
+    try state.update(alloc, &t);
+
+    const expected_viewport_y: usize = viewport_rows - tail_rows;
+    // BUG: This crashes without the fix
+    var cells = try state.linkCells(alloc, .{
+        .x = 0,
+        .y = expected_viewport_y,
+    });
+    defer cells.deinit(alloc);
+    try testing.expectEqual(@as(usize, 4), cells.count());
+}
+
 test "dirty row resets highlights" {
     const testing = std.testing;
     const alloc = testing.allocator;