Extension Manager UI behaves badly when multiple jobs are triggered

In order to reproduce the issue you need to trigger an extension job that takes some time (e.g. install an extension with many dependencies) and then, while this jobs is executed, trigger other jobs (install/uninstall/upgrade other extensions). The behaviour is inconsistent. Sometime the extension display is reloaded by the job is not triggered. Sometimes the extension display gets replaces by Velocity code (e.g. #displayExtension($extension)). Other times the job is triggered but the progress bar is at 100% although the job didn't even started.