terminal: fix off-by-one tracked pin issues when page is pruned

src/terminal/PageList.zig

@@ -1962,7 +1962,7 @@ pub fn pointFromPin(self: *const PageList, tag: point.Tag, p: Pin) ?point.Point
         if (tl.y > p.y) return null;
         coord.y = p.y - tl.y;
     } else {
-        coord.y += tl.page.data.size.rows - tl.y - 1;
+        coord.y += tl.page.data.size.rows - tl.y;
         var page_ = tl.page.next;
         while (page_) |page| : (page_ = page.next) {
             if (page == p.page) {
@@ -2879,6 +2879,43 @@ test "PageList pointFromPin active from prior page" {
     }
 }
 
+test "PageList pointFromPin traverse pages" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var s = try init(alloc, 80, 24, null);
+    defer s.deinit();
+    const page = &s.pages.last.?.data;
+    for (0..page.capacity.rows * 2) |_| {
+        _ = try s.grow();
+    }
+
+    {
+        const pages = s.totalPages();
+        const page_cap = page.capacity.rows;
+        const expected_y = page_cap * (pages - 2) + 5;
+
+        try testing.expectEqual(point.Point{
+            .screen = .{
+                .y = expected_y,
+                .x = 2,
+            },
+        }, s.pointFromPin(.screen, .{
+            .page = s.pages.last.?.prev.?,
+            .y = 5,
+            .x = 2,
+        }).?);
+    }
+
+    // Prior page
+    {
+        try testing.expect(s.pointFromPin(.active, .{
+            .page = s.pages.first.?,
+            .y = 0,
+            .x = 0,
+        }) == null);
+    }
+}
 test "PageList active after grow" {
     const testing = std.testing;
     const alloc = testing.allocator;

src/terminal/Screen.zig

@@ -479,10 +479,35 @@ pub fn cursorDownScroll(self: *Screen) !void {
             page_pin.page.data.getCells(self.cursor.page_row),
         );
     } else {
+        const old_pin = self.cursor.page_pin.*;
+
         // Grow our pages by one row. The PageList will handle if we need to
         // allocate, prune scrollback, whatever.
         _ = try self.pages.grow();
-        const page_pin = self.cursor.page_pin.down(1).?;
+
+        // If our pin page change it means that the page that the pin
+        // was on was pruned. In this case, grow() moves the pin to
+        // the top-left of the new page. This effectively moves it by
+        // one already, we just need to fix up the x value.
+        const page_pin = if (old_pin.page == self.cursor.page_pin.page)
+            self.cursor.page_pin.down(1).?
+        else reuse: {
+            var pin = self.cursor.page_pin.*;
+            pin.x = self.cursor.x;
+            break :reuse pin;
+        };
+
+        // These assertions help catch some pagelist math errors. Our
+        // x/y should be unchanged after the grow.
+        if (comptime std.debug.runtime_safety) {
+            const active = self.pages.pointFromPin(
+                .active,
+                page_pin,
+            ).?.active;
+            assert(active.x == self.cursor.x);
+            assert(active.y == self.cursor.y);
+        }
+
         const page_rac = page_pin.rowAndCell();
         self.cursor.page_pin.* = page_pin;
         self.cursor.page_row = page_rac.row;
@@ -2745,14 +2770,15 @@ test "Screen: scrolling when viewport is pruned" {
 
     // Our viewport is now somewhere pinned. Create so much scrollback
     // that we prune it.
+    try s.testWriteString("\n");
     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.expectEqualStrings("1ABCD\n2EFGH\n3IJKL", contents);
     }
 
     {