Usually this is not visible with only the Default resource pack, but loading certain resource packs can cause extra pixels to be incorrectly visible next to the jump bar when it is full until the game is restarted.

Steps to reproduce:

1. Go into the resource pack menu and load the resource pack (see attached test_resource_pack.zip).
2. Go back in into the resource pack menu and remove it again: load only Default.
3. Ride a tamed horse and hold the space bar. Note that when the jump bar is completely full, there are now weird pixels on the right of it that shouldn't be there.
   

Note that if you press F3+S, you can see that these extra pixels are from the minecraft_textures_atlas_gui.png_0 atlas having leftover pixels from the previously loaded resource pack in areas that should be unused. That itself isn't (presumably) a bug, however it makes it clear that the jump bar progress texture is being rendered with incorrect UVs from the atlas.

The resource pack I attached is designed to reproduce this issue, but in rare cases it can affect and confuse unsuspecting resource pack creators trying to make "actual" texture packs.