Debugging ​
Since all logic runs in ABAP, you can debug everything in the ABAP environment. Set an external breakpoint, because abap2UI5 apps are called externally via HTTP.
Backend ​
Set a breakpoint in your abap2UI5 app to debug the code. Check that the XML view builds correctly and that all events fire on the backend as expected.
Frontend ​
On the frontend, abap2UI5 behaves like a standard UI5 app, so the usual tools and debugging features work.
Developer Tools ​
Press Ctrl+F12 to open the built-in Developer Tools of abap2UI5:
Tabs cover the whole roundtrip: Error and Log, the System info, the Previous Request and Response, the app's Source Code, and for every view slot (main, popup, popover, nested) the rendered View XML and its Model data.
The footer offers Logout, Restart, a jump to ADT, and an Export that bundles everything — including the running app's ABAP class source — into one blob you can attach to a bug report. Error popups also carry a copy-to-clipboard button for the same purpose.
UI5 Inspector ​
Another option: the SAP default debugging tool, the UI5 Inspector.
Issues ​
If your code looks correct but you suspect a bug in the abap2UI5 framework, build a minimal sample that reproduces the bug and open an issue on the abap2UI5 repository — we'll investigate and fix it.