bl_info = {
    "name": "Keep Active Tool Undo",
    "version": (1, 0),
    "blender": (2, 90, 0),
    "category": "Object",
}
import bpy

class KeepActiveToolUndo(bpy.types.Operator):
    """Keep Active Tool Undo"""
    bl_idname = "wm.keep_active_tool_undo"
    bl_label = "Keep Active Tool Undo"

    def execute(self, context):
        
        # record active tool and mode
        original_mode = context.mode
        original_tool_id = context.workspace.tools.from_space_view3d_mode(original_mode, create=False).idname
        print("mode: "+original_mode)
        print("tool id: "+original_tool_id)
        
        done=False
        current_mode = original_mode
        current_tool_id = original_tool_id

        # keep trying to undo until the tool id hasn't changed after the undo
        while done==False:
            # undo
            try:
                print("undo")
                bpy.ops.ed.undo() #this line fails if undo stack is empty
            except:
                print("FAILED TO UNDO")
                self.report({'INFO'}, 'Failed to undo')
            # check new active tool and mode
            current_mode = context.mode
            current_tool_id = context.workspace.tools.from_space_view3d_mode(current_mode, create=False).idname
            print("mode: "+current_mode)
            print("tool id: "+current_tool_id)
            if original_tool_id == current_tool_id:
                done=True
                print("done")
                
            
        # change active tool back to the one we were on when this operator was called
        if original_tool_id != current_tool_id:
            print("tool id different **")
            print("")
            for area in context.screen.areas:
                if area.type == "VIEW_3D":
                    override = context.copy()
                    override["space_data"] = area.spaces[0]
                    override["area"] = area
                    bpy.ops.object.mode_set ( mode = original_mode )
                    bpy.ops.wm.tool_set_by_id(override, name=original_tool_id)

        print("done --------------------------------------") 
        print("")   
        return {'FINISHED'}

# store keymaps here to access after registration
addon_keymaps = []

def register():
    bpy.utils.register_class(KeepActiveToolUndo)
    wm=bpy.context.window_manager
    kc=wm.keyconfigs.addon
    if kc:
       km=kc.keymaps.new(name = "Window",space_type='EMPTY', region_type='WINDOW')
       kmi = km.keymap_items.new("wm.keep_active_tool_undo", type='Z', value='PRESS',oskey=True)
       addon_keymaps.append((km,kmi)) 

def unregister():
    for km,kmi in addon_keymaps:
        km.keymap_items.remove(kmi)
        addon_keymaps.clear()
    bpy.utils.unregister_class(KeepActiveToolUndo)

if __name__ == "__main__":
    register()