slideshowIslandRegex scanned the full file content on every editingFile
change even for the common non-slideshow case. Gate it behind a plain
substring check on SLIDESHOW_ISLAND_TYPE first — cheap, and avoids the
full-content RegExp pass for files that plainly have no island.
Added a test for the still-open behavior this preserves: a malformed
island (invalid JSON) still trips the substring check and the regex,
so the tab stays discoverable rather than silently disappearing.
The Slideshow tab rendered unconditionally, showing the branching editor
for any composition regardless of whether it was actually a slideshow —
a plain video comp offered a tab with nothing meaningful to edit.
Gate it on the composition carrying the slideshow JSON island
(<script type="application/hyperframes-slideshow+json">), the same
definitive marker the CLI's `present` command already requires (it
refuses to run without one). Presence-only, not full manifest
validation, so a malformed island still surfaces the tab rather than
disappearing entirely. Also bounce rightPanelTab off "slideshow" to
"renders" if the active composition stops being a slideshow while that
tab is open (e.g. switching files), since its button would otherwise
vanish with no way back to it.
Extracted the gating + scene-list derivation into useSlideshowTabState
to keep StudioRightPanel.tsx under the 600-LOC gate.
Vance Ingalls