What's this gap between the right end of the mode-line and the fringe when changing font's size?

Question

I'm toying around with font sizes in Emacs and I noticed that changing the font size to something huge, like, 160, creates a gap between the right end of the mode-line and the fringe:

This is the code I ran on my scratch buffer:

(let ((faces '(mode-line
               mode-line-buffer-id
               mode-line-emphasis
               mode-line-highlight
               mode-line-inactive)))
  (set-face-attribute 'default nil :height (* 16 10))
  (mapc (lambda (face) (set-face-attribute face nil :height (* 9 10))) faces))

Yes, I'm trying to keep the mode-line font size I currently use (900)

Answer 1

The mode-line gets filled from left to right. Anything in the mode-line that is clearly skewed to the right is most likely occurring because a filler has been used -- e.g., read-only column 22 line 13 filler filler filler filler stuff-on-right.

If the filler is based on a particular font that becomes smaller, or if certain textual components of the mode-line become smaller due to a font change or font size reduction, then there may appear to be extra space on the right. The original poster should research the contents of the mode-line-format to see what else is present, and modify those aspects accordingly so that certain aspects appear to be better right-aligned (using additional filler).

Or, like many users, the original poster could try out the default behavior of the mode-line without using any filler.