I've looked around and every time someone mentions diff tools in context of TortoiseHG, they mean the external diff tool. But I'd like to change the internal TortoiseHG Workbench GUI app's diff tool to something better, and I couldn't find any results for that.
The documentation has this image and briefly describes it without saying if it's even possible to customize a diff tool for this panel (bottom right):
Changing the default diff tool only affects which external app is used. This built-in diff view is not affected by that setting. But maybe there is a hidden setting which can be explicitly typed into the config?