The ornamental design for a height-adjustable armrest, as shown and described.
FIG. 1 is a perspective view of the height-adjustable armrest showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a rear side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a left side view thereof; and,
FIG. 8 is an isometric view illustrating the position of the height-adjustable armrest on a chair.
The height-adjustable armrest features a button and hollow at the top of the armrest as shown in the drawings. The height of the armrest can be adjusted when the button is pressed.
The broken line showing of a chair is included for the purpose of illustrating environmental structure and forms no part of the claimed design.