Quick answer: Unity legacy EventSystem stealing input events from UI Toolkit panels? Two input handlers compete - disable legacy EventSystem when UI Toolkit is the UI of record.
Migrated UI to UI Toolkit. Buttons don't respond because the leftover EventSystem swallows the click events.
Remove legacy EventSystem
Delete from the scene. UI Toolkit has its own event dispatch; no EventSystem required.
Or disable UI Toolkit panel pick
If you must keep both, set pickingMode = Ignore on conflict-prone elements. Surgical; works.
Choose one input system
Per project: legacy uGUI or UI Toolkit. Mixing is bug-prone.
“Two UI systems compete for input. Compete = one of them loses.”
For new projects, UI Toolkit. Migrate legacy uGUI gradually; do not run both for long.