The hotbar texture is calculated to be 182 wide and 22 high from an origin of 0x 0y on the widgets.png. This matches the size present in the vanilla widgets.png image.

The selector is currently calculated to be 24 wide and 22 high from an origin of 0x 22y, just below the hotbar. This does not match the texture, which is 22 wide and 24 high.

View below a blown up image of a portion of the hotbar, 22 pixels tall, and the selector below it, 24 pixels tall, in both vanilla and resource pack assets.

Not only does this cut off a single visible pixel from the selector with the current hotbar placement, it technically cuts off two since the hotbar (as present in VANILLA assets) would have to be moved up a pixel from the bottom of the screen to accommodate the vanilla selector asset design. The hotbar is placed 22 from the screen bottom. This works for the hotbar itself alone, but not when considering the selector. The selector is placed 23 from the screen bottom, in alignment with the hotbar. Design-wise it should stick out one pixel above and below the hotbar. So while the two bottom pixels are cut off from selecting which pixels from the texture to grab to render the selector (22 of 24 pixels of height), only one row of pixels is shown to not render on the screen.

While a solution of making the selector calculate to 23 pixels in height would mask up the cut portion on the screen space, a full fix would require making the selector 24 pixels tall, placing it 24 pixels from the bottom of the screen, and placing the hotbar 23 pixels from the bottom of the screen (and as well moving other GUI elements up by a pixel).

This isn't too noticeable with vanilla assets as the bottom of the hotbar is opaque, but with a resource pack it becomes quite glaring. You can see where the selector is cut off compared to how it should be in the texture file, above.

This was reported in MC-75406 but inaccurately marked as resolved.