[FEATURE] Artifact design canvas: support Shift + mouse wheel for horizontal panning

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

On a published design canvas, the mouse wheel maps to two of the three expected
gestures but not the third:

• wheel → pans vertically ✅
• Ctrl + wheel → zooms ✅
• Shift + wheel → does nothing ❌

Shift + wheel is the near-universal convention for horizontal scrolling on a
mouse (browsers, VS Code, Figma, Excel, most canvas tools), so its absence reads
as a broken input rather than a missing feature. The only way to move sideways
today is click-and-drag on empty canvas, which is a problem in exactly the case
the canvas is built for: a wide, multi-artboard layout. On a canvas ~5000 px
wide, reaching the rightmost artboard takes several drag-release cycles, each of
which requires first finding empty space to grab — hard when artboards are laid
out in rows with only 80–120 px between them. Trackpad users get two-finger
horizontal panning for free; mouse users, who are the majority on desktop
workstations, have no equivalent.

Proposed Solution

Map Shift + wheel to horizontal panning of the canvas viewport:

• deltaY under Shift moves the viewport on the X axis, with the same speed and

easing as the existing vertical pan.

• Also honour a real horizontal wheel signal (deltaX ≠ 0) for tilt wheels and

horizontal-capable mice, which is the same code path.

• Keep Ctrl + wheel (zoom) taking precedence when both modifiers are held.
• Call preventDefault() so the host page does not scroll behind the canvas.

Alternative Solutions

• Middle-click / space-drag panning — helps, but is a second gesture to

learn and still requires a held button; it does not remove the need for the
conventional one.

• Horizontal scrollbar on the canvas — discoverable, but costs screen space

and is slow for long distances.

• Minimap or "fit to view" shortcut — useful for big jumps, but does not

help with the frequent small sideways nudges while comparing two artboards
side by side.

• Keyboard arrows to pan — a good accessibility addition regardless, but not

a substitute for a wheel gesture while the other hand is on the mouse.

Priority

Medium - Would be very helpful

Feature Category

Other

Use Case Example

• A design canvas holds 12 artboards laid out in rows spanning roughly 5000 canvas px.
• The reviewer compares the "list screen" artboard with the "contact detail" artboard sitting to its right.
• They scroll down to the row they want (wheel) and zoom to a readable level (Ctrl + wheel) — both work.
• They then hold Shift and scroll to slide one artboard to the left. Nothing moves.
• They must let go of the wheel, find empty canvas between two artboards, click, drag, release, and repeat, several times per comparison.
• With Shift + wheel mapped, the same movement is one gesture without leaving the wheel — the same way every other canvas tool behaves.

Additional Context

Latest artifact url, chrome, win 11: https://claude.ai/code/artifact/