Symptom:

After the apt-get dist-upgrade last night, the characters in the  system title bar became all the squares.

Reason:

To check out the fonts configuration, we can use

sudo apt-get install dconf-tools

The issue command dconf-editor

Then expand org>gnome>desktop>wm>preferences: then choose the “titlebar-font”, first of all, read the Default , in my case it was “Cantarell Bold 11”. Then I opened the tweak tool, search the “Cantarell” and found that the font was corrupt.

Solotion:

To solve this, I decide to reinstall the font and see if it works.

In linux, we can install or remove the font by apt-get command, so:

remove the Cantarell font: apt-get purge fonts-cantarell

Install it: apt-get install fonts-cantarell

And the system will configure the font automatically.