
Master (413e438ea9767710d4810c4400fd1242ca52cd1c) on Bionic + Qt 5.12.4.
Steps to reproduce the issue:
- Window -> Information
- close debug window
- Window -> Information

Master (413e438ea9767710d4810c4400fd1242ca52cd1c) on Bionic + Qt 5.12.4.
Steps to reproduce the issue:
Do you think this is more likely a regression in Qt 5.12.4, or because you're running on bionic? I cannot reproduce with Qt 5.12.3 on macOS. Will setup a Bionic VM to try recreate.
I setup a new Bionic VM (18.04.2 LTS), installed Qt 5.12.4 LTS using the online installer and was able to recreate. Note that resizing the debug window will show the correct contents.

Tested Qt 5.12.0. No bug. Yes, it seems a regression in Qt 5.12.4.
Something like QTBUG-76509.
I setup a new Bionic VM (18.04.2 LTS), installed Qt 5.12.4 LTS using the online installer and was able to recreate.
What ./configure arguments do you use in such setup?
Something like QTBUG-76509.
Yea that looks like what it could be.
What ./configure arguments do you use in such setup?
I just used PKG_CONFIG_PATH=/path/to/Qt/5.12.4/gcc_64/lib/pkgconfig ./configure --with-incompatible-bdb, and then used LD_PRELOAD at runtime to specify the .sos at runtime.
I've read some of your related notes + discussion upstream in the Qt bug tracker about issues with the online installer. I'm going to write up some notes for myself in core-review.
Tested Qt 5.13.0. No bug.