Tap tool default mode should select+unselect, but not append about bokeh

[mattpap]

Since bokeh 3.4 the default for tap tool is xor mode. It may appear to look like the tool is in append mode, but in append mode the user can only add selected data points, but not remove them, which the main benefit of xor mode. We switched to xor default, because users wanted to be able to deselect points and especially toggle data points.

The case from discourse the prompted this discussion silently assumes replace mode, which now requires explicit specification. The best way, that works with all modes, would be to keep track of selected data points and perform actions only for newly selected data points. Currently there is no built-in functionality for this.

[bryevdv]

cc @bokeh/dev I think this change should be re-visited. It certainly makes sense to offer this mode for some niche cases, but I don't think it makes sense as the default (and it clearly defies some user's expectations as well as seen on the discourse). This is especially jarring in plots like pie or bar charts where there are only a handful of elements to select. This also puts one tool on different footing than all other tools which is not good for simple documentation. Many users have used tap selection as part of a larger UI, e.g. to trigger some other drill down or other UI update and this change breaks all of those usages.

[bryevdv]

FWIW I would be ok with this default behavior:

• tapping new points replaces the existing selection (previous behavior)
• tapping an already selected point unselects it (new behavior)

i.e. it is very specifically the "additive" nature of this xor-mode, where tapping a new point adds to an existing selection, that is the problem

[philippjfr]

I agree with that proposal. Let's allow deselect but not adding by default.

[mattpap]

Do we want this behavior to be implemented in place of replace mode (just for tap tool) or should I introduce a new mode?

[bryevdv]

I actually don't know what all the current modes are to say, where are they enumerated/described?

Well, I guess they are listed here

SelectionMode = Enumeration(replace, append, intersect, subtract, xor)

I guess it's a little weird, I don't think one set of selection modes really neatly covers all the tools in the same way. i.e the re-select to un-select that can make sense for a tap is nearly impossible to imagine being useful on a lasso tool. I think I lean towards: new mode, e.g. replace-foo with that being the default for tap. That's in the interest of not having "replace" mean multiple things, but also being able to say "a replace mode" is the default for all tools, which is less awkward to explain. But I am interested in others' thoughts.