Observation Details
Opening the filter dialog window can obscure underlying focused elements (partially AND completely). Since the dialog window cannot be closed via ESC in that situation, the underlying element cannot be made visible without removing the keyboard focus from it.
Remediation Notes
Dialog windows must be made accessible. They must trap the keyboard focus when opened and must not allow scrolling of the page and focusing of underlying interactive elements. They must be closable via ESC and the close button must be operable by keyboard as well. This will remediate the obscuring of focused elements.