File open dialog and similar not adaptive anymore with newer versions of Purism's gtk fork

To resolve this properly, we'll probably need to do a bisect.

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Describe your issue

What's the expected behaviour?

Dialogs like File Open, About, ... fit the mobile display.

What's the current behaviour?

They look just like on the desktop, and the phone can only display them partially, making them unusable.

How to reproduce your issue?

• Install the version from !1810. As of writing, this is current master, but I'll revert it with !1835 today.
• I've also tried the most recent version as of writing, https://source.puri.sm/Librem5/debs/gtk/-/commit/261616199e6229d51762007f3cf35575efe56856, and it shows the same issue on postmarketOS.

What device are you using?

pine64-pinephone

On what postmarketOS version did you encounter the issue?

• edge (master branch)
• stable (v20.05 branch)

On what environment did you encounter the issue?

Phone environments

• Phosh

How did you get postmarketOS image?

• from https://images.postmarketos.org
• I built it using pmbootstrap
• It was preinstalled on my device

What's the build date of the image? (in yyyy-mm-dd format)

2021-01-25