A new hadronic calorimeter (HCAL) with glass scintillator tiles has been designed for future lepton collider experiments (e.g. the Circular Electron Positron Collider). Using a sampling structure (similar to the CALICE AHCAL technology), the new HCAL design aims for better hadron and jet performance, with a higher sampling fraction by using high-density glass scintillator instead of plastic scintillator. Full simulation studies were done on jet performance of Higgs hadronic decays using a Particle-Flow Algorithm (PFA). The HCAL design was optimised in terms of longitudinal depth, transverse granularity, glass density and effective light yield. Hardware activities focus on measurements of high-density glass tiles developed within the Glass Scintillator Collaboration. First two batches of glass tiles in the target dimensions of $\mathrm{40\times40\times10~mm^{3}}$ were tested with beam particles at CERN and DESY. In the contribution highlights of R\&D activities will be presented, including performance studies, design optimisations and latest beam test results.