terminal: pruned pages should keep tracked pins in top-left

src/terminal/PageList.zig

@@ -1571,6 +1571,17 @@ pub fn grow(self: *PageList) !?*List.Node {
         first.data.size.rows = 1;
         self.pages.insertAfter(last, first);
 
+        // Update any tracked pins that point to this page to point to the
+        // new first page to the top-left.
+        var it = self.tracked_pins.keyIterator();
+        while (it.next()) |p_ptr| {
+            const p = p_ptr.*;
+            if (p.page != first) continue;
+            p.page = self.pages.first.?;
+            p.y = 0;
+            p.x = 0;
+        }
+
         // In this case we do NOT need to update page_size because
         // we're reusing an existing page so nothing has changed.
 
@@ -3271,6 +3282,11 @@ test "PageList grow prune scrollback" {
     // Get our page size
     const old_page_size = s.page_size;
 
+    // Create a tracked pin in the first page
+    const p = try s.trackPin(s.pin(.{ .screen = .{} }).?);
+    defer s.untrackPin(p);
+    try testing.expect(p.page == s.pages.first.?);
+
     // Next should create a new page, but it should reuse our first
     // page since we're at max size.
     const new = (try s.grow()).?;
@@ -3280,6 +3296,11 @@ test "PageList grow prune scrollback" {
     // Our first should now be page2 and our last should be page1
     try testing.expectEqual(page2_node, s.pages.first.?);
     try testing.expectEqual(page1_node, s.pages.last.?);
+
+    // Our tracked pin should point to the top-left of the first page
+    try testing.expect(p.page == s.pages.first.?);
+    try testing.expect(p.x == 0);
+    try testing.expect(p.y == 0);
 }
 
 test "PageList adjustCapacity to increase styles" {

src/terminal/Screen.zig

@@ -2706,6 +2706,63 @@ test "Screen: scrolling moves selection" {
     }
 }
 
+test "Screen: scrolling moves viewport" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var s = try init(alloc, 10, 3, 1);
+    defer s.deinit();
+    try s.testWriteString("1ABCD\n2EFGH\n3IJKL\n");
+    try s.testWriteString("1ABCD\n2EFGH\n3IJKL");
+    s.scroll(.{ .delta_row = -2 });
+
+    {
+        // Test our contents rotated
+        const contents = try s.dumpStringAlloc(alloc, .{ .viewport = .{} });
+        defer alloc.free(contents);
+        try testing.expectEqualStrings("2EFGH\n3IJKL\n1ABCD", contents);
+    }
+
+    {
+        try testing.expectEqual(point.Point{ .screen = .{
+            .x = 0,
+            .y = 1,
+        } }, s.pages.pointFromPin(.screen, s.pages.getTopLeft(.viewport)));
+    }
+}
+
+test "Screen: scrolling when viewport is pruned" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var s = try init(alloc, 215, 3, 1);
+    defer s.deinit();
+
+    // Write some to create scrollback and move back into our scrollback.
+    try s.testWriteString("1ABCD\n2EFGH\n3IJKL\n");
+    try s.testWriteString("1ABCD\n2EFGH\n3IJKL");
+    s.scroll(.{ .delta_row = -2 });
+
+    // Our viewport is now somewhere pinned. Create so much scrollback
+    // that we prune it.
+    for (0..1000) |_| try s.testWriteString("1ABCD\n2EFGH\n3IJKL\n");
+    try s.testWriteString("1ABCD\n2EFGH\n3IJKL");
+
+    {
+        // Test our contents rotated
+        const contents = try s.dumpStringAlloc(alloc, .{ .viewport = .{} });
+        defer alloc.free(contents);
+        try testing.expectEqualStrings("2EFGH\n3IJKL\n1ABCD", contents);
+    }
+
+    {
+        try testing.expectEqual(point.Point{ .screen = .{
+            .x = 0,
+            .y = 0,
+        } }, s.pages.pointFromPin(.screen, s.pages.getTopLeft(.viewport)));
+    }
+}
+
 test "Screen: scroll and clear full screen" {
     const testing = std.testing;
     const alloc = testing.allocator;