Xournalpp freezes when changing following page size from A4 to A5

[ethpi]

Affects versions :

• OS: Kubuntu 18.04.3 LTS
• KDE/PLASMA shipped with Kubuntu 18.04.3 LTS
• libgtk 3.0 (3.22.30-1ubuntu4)
• Version of Xournal++ 1.1.0+dev

Describe the bug
xournalpp window freezes, when changing the page size from a following page to a smaller format.

To Reproduce
Steps to reproduce the behavior:

1. Open new document.
2. Insert a second page.
3. Change the second page size from e.g. A4 to A5.

Expected behavior
xournal++ window freezes, after about 30s it terminates with the console error message
"Speicherzugriffsfehler (Speicherabzug geschrieben)"

Shall I attach the core dump? It's a file of more than 5MB! There is no error log made by xournalpp.

Additional context
With the version 1.0.16 it's no problem to create a document with a second smaller page.
I have a few journals created with a 1.0.x version, that I can't open with 1.1.0+dev. When I try to open these old journals, the xournalpp window freezes.
When I unzipped these journals, changed all lines containing

<page width="611.99998270" height="791.99998270">

to

<page width="595.27000000" height="841.89000000">

then I can reopen these old journals with the 1.1.0+dev version. The freezing seems to belong to different page sizes.

[Technius]

Appears to be a stack overflow caused by ZoomControl: changing the paper size calls ZoomControl::pageSizeChanged, which leads to the zoom value getting updated, which triggers the scroll value getting updated, which then triggers a page selection, which then triggers the zoom value getting updated again, which triggers the scroll value getting updated...

I'm guessing this is a regression due to 6c2b7a7.

[Technius]

A temporary user workaround is to disable zoom fit mode (the icon with the square corner cutouts).

Edit: in other words, this bug only occurs when zoom fit is enabled.

[Technius]

Very closely related to #1608.

[Technius]

Instead of waiting for the viewport rewrite (e.g. the "perfect" solution), I'll see if I can get a hotfix for this out this week...