Ok - I have completed my testing.
1. We get different results immediately after changing scaling using the dropdown compared to then sign-out/in or rebooting. When you change the dropdown the immediate consequence is similar to changing the screen resolution and the mIRC dialog looks fine - but then when you reboot you get genuine scaling and the dialog breaks.
The change of resolution after changing the dropdown is IMO an anomaly.
2. Screen shots of all of the above suggest that this is a general issue with mIRCs handling of scaling other than 100%.
On my system these are the values I get from:
//echo Screen resolution: Width: $window(-1).h Height: $window(-1).h / DBU: Width: $dbuw Height: $dbuh
100% Width: 1920 Height: 1080 / DBUW: 2 DBUH: 2
125% Width: 1920 Height: 1080 / DBUW 2.5 DBUH 2.5
150% Width: 1920 Height: 1080 / DBUW 2.5 DBUH 2.5
175% Width: 1920 Height: 1080 / DBUW 2.5 DBUH 2.5
Here are the screen shots: