Watch the side line, these parts are more opaque or you can say darker but the alpha in hud_screen.json is written as 0.65, this should not happen:

Luckily the images in vanilla is all black pixels, which will not expose the bug obviously in vanilla, it just darker.

However the wrose is when you modify the images "hotbar_start_cap.png" or "hotbar_end_cap.png" by resourse pack, when the two images are not totally black, there will be rendered weirder:

For example:

Quadral Texture Pack

Plastic Texture Pack

Especially in HD resource pack including the two images within thiner lines:

Faithful 32x Resource Pack

I was confused by this when I was making a texture pack but I found the reason now.

In vanilla resource pack, there are two JSON files were written by mistake: 

"hotbar_start_cap.json" & "hotbar_end_cap.json", the two files should never happen because they are used to describe the base size and nineslice size of images. However the "hotbar_start_cap.png" and "hotbar_end_cap.png" are not need to expand the scale in game rendering. So the two files are redundant.

At the same time the redundant files were written by mistake, which caused the bug, they were written as:

{
  "nineslice_size": [
    1,
    1,
    1,
    1
  ],
  "base_size": [
    1,
    1,
    1,
    1
  ]
}

Two files are same.
As we can see the base_size is [ 1, 1, 1, 1 ]. However an image can only use two number for length and width, these are 2D images, not 4D! And the nineslice_size part is also redundant.

For developer to fix this bug, just remove the two JSON files.

For resource pack maker, there is a correct JSON code you can copy into the two files in your pack to fix the bug:

{
  "base_size": [
    1,
    22
  ]
}