Fix viewport going out of bounds when zooming out (#7899)

The viewport pin was allowed to go below the active pin during resize,
which caused a use of null when trying to get the bottom right of the
viewport since going down by the active row count from the viewport pin
ends up below the bottom of the active area.

Ran in to this while doing font work because I was scrolled up slightly
and increasing and decreasing to see stuff at different sizes.

src/terminal/PageList.zig

@@ -1401,6 +1401,15 @@ fn resizeWithoutReflow(self: *PageList, opts: Resize) !void {
                     assert(count < rows);
                     for (count..rows) |_| _ = try self.grow();
                 }
+
+                // Make sure that the viewport pin isn't below the active
+                // area, since that will lead to all sorts of problems.
+                switch (self.viewport) {
+                    .pin => if (self.pinIsActive(self.viewport_pin.*)) {
+                        self.viewport = .{ .active = {} };
+                    },
+                    .active, .top => {},
+                }
             },
         }
 
@@ -5975,6 +5984,36 @@ test "PageList resize (no reflow) more rows extends blank lines" {
     }
 }
 
+test "PageList resize (no reflow) more rows contains viewport" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    // When the rows are increased we need to make sure that the viewport
+    // doesn't end up below the active area if it's currently in pin mode.
+
+    var s = try init(alloc, 5, 5, 1);
+    defer s.deinit();
+    try testing.expect(s.pages.first == s.pages.last);
+
+    // Make it so we have scrollback
+    _ = try s.grow();
+
+    try testing.expectEqual(@as(usize, 5), s.rows);
+    try testing.expectEqual(@as(usize, 6), s.totalRows());
+
+    // Set viewport above active by scrolling up one.
+    s.scroll(.{ .delta_row = -1 });
+    // The viewport should be a pin now.
+    try testing.expectEqual(Viewport.pin, s.viewport);
+
+    // Resize
+    try s.resize(.{ .rows = 7, .reflow = false });
+    try testing.expectEqual(@as(usize, 7), s.rows);
+    try testing.expectEqual(@as(usize, 7), s.totalRows());
+    // The viewport should now be active, not a pin.
+    try testing.expectEqual(Viewport.active, s.viewport);
+}
+
 test "PageList resize (no reflow) less cols" {
     const testing = std.testing;
     const alloc = testing.allocator;