Thanks for the report.  I'm unable to reproduce this issue on Linux, Mac, M69, M72.

On the surface, this issue looks similar to crbug.com/797352 where font sizes were not correctly applied 'sometimes' when Device Mode is enabled.  In the other report, it suggested adding width=device-width in the viewport descriptor:
  <meta name="viewport" content="width=device-width">

skobes@, maybe you have ideas why this might occur?

From the screenshots, this looks like an antialiasing issue, not a font size issue.  Maybe mobile emulation applies different antialiasing settings?

<meta name="viewport" content="width=device-width"> didnt fixed it for me.
It could be a antialiasing  issue (is there a way to check it?).
But if mobile antialiasing is different to the default antialiasing, it should switch back when leaving the developer tools (even when i dont close the device toolbar).

I mean, the antialiasing is the only difference I see between chrome_correct.png and chrome_broken.png, so I'm assuming that's what you're referring to when you say the "Font-Rendering is now broken".  But I don't know the root cause of the bug.

@luoe have you tried to repro on Windows?

Yes, I'm able to reproduce on a Win10 computer.  Having antialiasing change when entering mobile emulation is expected, but when exiting Device Mode, I'd expect that the antialiasing returns to "normal/desktop".

If we can get the normal aliasing to be equivalent to the [emulate mobile + unemulate mobile] case, that'd be less surprising to me.  I'll take a look at the emulator code.

Thanks!