An alternative approach would be to create a new userpref for non-primary mouse buttons, but IMO mouse 4+ fall well within "less common types" covered by the generic drag threshold.

Thanks for the patch!

This currently makes it so that you have to move the mouse 30px so the action is executed. Couldn't we just add a few pixel extra for that case? We could even just double it and clamp by U.drag_threshold.
Would like to hear what @William Reynish (billreynish) has to say about this.

(Note: code here was recently moved to a new file, wm_event_query.c, so this patch will have to be updated.)

I find it a little confusing - in what context is the current behavior an issue, and how does this address it?

I'm not keen on this change unless many users are finding this a problem.

Having the additional buttons on the mouse using a different drag-threshold depends on the physical design of the mouse.

Personally I use thumb button on my mouse and don't find the drag threshold to be a problem.

In D5770#159279, @William Reynish (billreynish) wrote:

I find it a little confusing - in what context is the current behavior an issue, and how does this address it?

For me this is an issue popped up while stepping through file browser navigation history via front/back buttons which would sporadically do nothing. These buttons (4th and 5th) are normally side-mounted on 5 button mice and thus more likely to cause a bit of travel when pressed. This change causes the drag threshold for buttons 4+ to fall through to the more conservative default drag threshold.

In D5770#159280, @Campbell Barton (campbellbarton) wrote:

Having the additional buttons on the mouse using a different drag-threshold depends on the physical design of the mouse.

I think it's fair to say side mounting thumb buttons is increasingly standard, as confirmed by an image search for 5 button mice. Exceptions are uncommon, similar to side mounted left/right mouse buttons (ergonomic mice).

In D5770#159280, @Campbell Barton (campbellbarton) wrote:

Personally I use thumb button on my mouse and don't find the drag threshold to be a problem.

I evaluated the potential impact of this change by checking against the default key configs, to which the only uses were the file navigation configs I had contributed some time ago. Are you referring to file browser navigation via thumb buttons or a non-standard keymap not being an issue? If a non-standard keymap the issue might be hidden by checking against a mouse event value of "Press" instead of "Click".

In D5770#159280, @Campbell Barton (campbellbarton) wrote:

I'm not keen on this change unless many users are finding this a problem.

Do you have thoughts on alternatives or do you think the added complexity doesn't support it's weight e.g. above I suggested an alternative might be another userpref for tertiary mouse buttons?