When entering text in the properties panel (eg. Tempo>Text, Dynamics>Suffix) it is currently not possible to enter any characters using Windows ALT-codes that contain a 0 such as an em-dash (— , ALT+0151)
I presume that Dorico is interpreting the ALT+0 and ALT+Num0 key combinations as a global shortcut which then causes the text box to lose focus, mid ALT-code.
These characters are obviously still supported by Dorico in these properties as it is possible to copy-paste them from elsewhere (including Shift-X text and popovers).
Can this be remapped/disabled? I’ve searched Preferences>Key Commands dialog for the relevant command to no avail.
Given that this conflicts with a global feature of the Windows OS, I’d suggest that Dorico either use a different default key binding, or at least disable it in when in “active” text boxes.
You can disable it manually, yes. You’ll need a decent text editor and a strong enough nerve to manually edit JSON files. (You can check that your edited JSON is still valid by pasting it into e.g. here.)
Look in %APPDATA%\Steinberg\Dorico 5 and find your keycommands_en.json file. Open it up in your favourite text editor and insert the following somewhere in the kGlobal context: