bug: half-width second-device placement and preview inconsistency in same RU

[ggfevans]

Summary

Half-width device placement in a single RU is inconsistent: after placing one half-width device in left/right slot, placing a second half-width device in the opposite empty slot can fail.

Problem

A user reported that half-width workflows do not reliably allow two devices in the same RU (left + right), which is required for shelf-style AV rack planning use cases.

Maintainer observation also indicates drag preview inconsistency for half-width devices (showing full-width and half-width boxes at the same time), suggesting slot targeting/preview state may be out of sync.

Steps to Reproduce

1. Place a half-width device in a rack RU.
2. Move/set it to left (or right) slot.
3. Attempt to place a second half-width device in the opposite slot of the same RU.
4. Observe placement result and drag preview visuals.

Expected Behavior

• Two half-width devices can occupy the same RU when they use opposite slots.
• Half-width drag preview should render a single correct half-width target indicator.

Actual Behavior

• Second device placement in the opposite slot may fail.
• Drag preview can show conflicting overlays (full-width + half-width) during half-width drag.

Scope

• Standard rack view placement/drag
• Slot position persistence through drop pipeline
• Half-width preview rendering consistency

Acceptance Criteria

• A second half-width device can be placed in the opposite empty slot of the same RU.
• Half-width drag preview shows only the correct half-width target indicator (no duplicate/conflicting full-width overlay).
• No regression for full-width device placement/preview behavior.
• Works consistently in views that support rack drops (standard + grouped/bayed paths, if applicable).

Test Requirements

• Add/adjust automated coverage for half-width slot position propagation through drop handlers.
• Add regression coverage for half-width drag preview state.
• Manual verification with the reported scenario (e.g., Intel NUC + Mac mini on same shelf RU).

Parent Epic

• epic: half-width UX audit and stabilization for v0.8.1 #1194

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Original Report (Preserved)

Discussed in #1145

Originally posted by marcuspee on February 9, 2026:

Adding half width racks or depths does not allow me to place multiple items in a single RU.
An example is placing an Intel NUC on the left, and a MacMini on the right, same rack shelf.

There are options for adding them as half width, and moving them within the RU to the left or right slot, but then I am unable to add a second device to the empty slot.

This would be key for planning AV Racks if possible!

Thank you!

[ggfevans]

Cannot repro in dev