Zoom in/out buttons should round zoom level

[1ec5]

NavigationControl’s Zoom In and Zoom Out buttons always increment and decrement the zoom level by exactly one, even if the initial zoom level isn’t an integer. For example, if the map is at zoom level 14.5 and you click Zoom In, the resulting zoom level is 15.5. This is inconsistent with the + and − keyboard shortcuts, which round the zoom level to the nearest integer. The Zoom In and Zoom Out buttons should likewise round the zoom level.

These buttons should behave consistently with the keyboard shortcuts, because some people are only able to use one or the other at any given time. They have the same plus and minus signs, so the user would have no reason to believe they’d behave differently. Of the two behaviors, style authors would likely appreciate rounding to the nearest integer. Zoom stops are only evaluated at integer zoom levels: mapbox/mapbox-gl-js#13291. Moreover, raster basemap tiles (as opposed to satellite imagery) easily appear blurry at fractional zoom levels, so users would want an easy way to get to something more legible.

[HarelM]

I think this is a good idea to unify between keyboard and zoom controls.
Since this is a behavior change I would recommend doing it in a breaking change version, which is tracked here:

• v6 breaking changes features #6427

[mizmay]

Anyone mind if I take a crack at this?

[neodescis]

Agreed it would be good to unify the behaviors. Just know that the + and - keyboard actions don't necessarily just go to the next integer value; it appears that the current value is rounded before going to the next value. In other words, if you're currently at zoom level 14.6, hitting the + button results in a zoom level of 16.

[mizmay]

Investigated. The +/- button behavior is controlled in src/ui/camera.ts:

this.getZoom() + 1 => zoom 14.6 becomes 15.6 when + button is pressed

The keyboard controller is at src/ui/handler/keyboard.ts:

zoom: zoomDir ? Math.round(tr.zoom) + zoomDir * (e.shiftKey ? 2 : 1) : tr.zoom, => zoom 14.6 becomes 16 when + key is pressed, or 17 with Shift-+

Noting this is already flagged as a breaking change.

Options:

1. Change the button behavior to match the keyboard behavior? Or
2. Align both to the idea that +/- should take you to the nearest zoom?

Starting Zoom	Action	Option 1 (Round then add)	Option 2 (Snap to integer)
14.6	+	16	15
14.6	Shift++	17	16
14.6	-	14	14
14.6	Shift+-	13	13

[mizmay]

Ok I tested Option 2, 👎 , makes sense for 14.6, but at 14.9 it feels strange to round only to 15. Moving forward with:

Success Criteria

1. Behavior matches keyboard shortcuts exactly, e.g.
2. At zoom 14.5, clicking Zoom In results in zoom 16 (not 15 or 15.5)
3. At zoom 14.5, clicking Zoom Out results in zoom 14 (not 13 or 13.5)
4. At zoom 14.4, clicking Zoom In results in zoom 15 (rounds down to 14, +1 = 15)
5. At zoom 14.6, clicking Zoom Out results in zoom 14 (rounds up to 15, -1 = 14)

But don't mind revising if further thought on how to align both is warranted.

[1ec5]

For what it’s worth, Rapid and iD follow option 1, so I’m used to that behavior. I think I’ve seen option 2 somewhere (can’t remember). It probably wouldn’t feel terrible in general, but I could imagine someone on a phone getting to a zoom level like 7.99, pressing ➕, and wondering why nothing happened.

[mizmay]

Ok I created a draft PR... gonna sleep on it and invite @1ec5 and others to think about this further.

Almost certainly the vast majority of Maplibre users are mashing on those +/- buttons all day every day and are blissfully unaware of the important but fairly edge case concerns you are raising. This IS a change for them. They might notice!

There's also the Shift+ and Shift- behavior with the keyboard shortcuts.... they have a power user vibe. It might be ok for casual users to leave things as is.

On the other hand, it's not a huge deal, people might not even notice.

[mizmay]

For what it’s worth, Rapid and iD follow option 1, so I’m used to that behavior. I think I’ve seen option 2 somewhere (can’t remember). It probably wouldn’t feel terrible in general, but I could imagine someone on a phone getting to a zoom level like 7.99, pressing ➕, and wondering why nothing happened.

Option 3 would be to change the logic in both cases to increment by at least 0.5z and round from where you end up.