File explorer visibly shifts left on folder open/close (followup to #112)

[tavateva]

Followup to #110 / PR #112. Minor, low priority.

Symptom

After PR #112 lands, expanding or collapsing a folder in the file explorer visibly shifts the panel contents to the left. Reported by @den-sq on merge review.

Suspected causes

Two candidates worth checking first:

1. Chevron visibility rule change. PR feat(file-explorer): lazy per-directory watchers + deferred teardown #112 shows a chevron on every folder (lazy folders start empty until first expand — old visibility rule would have hidden them). If layout logic reserves space based on has-children state, the width can flip when children populate.
2. Scrollbar reflow. Expanding populates the tree and can push it past the panel height, adding a vertical scrollbar and stealing horizontal pixels; collapsing removes it. Would explain the "shift back" on collapse if overflow: scroll (always reserve) vs overflow: auto (reserve on demand) is set inconsistently.

Repro

Fresh dev run, open the file explorer, expand any non-empty folder, then collapse it. Observe the contents jump horizontally.

Fix direction

Likely a small CSS change in DraggableEntry or the file-explorer panel container: either give the chevron column a fixed width regardless of children, or standardize on overflow-y: scroll / scrollbar-gutter: stable so the scrollbar space is always reserved.

Scope: cosmetic only, doesn't affect functionality. Filing so it doesn't get lost.