The box was hidden until an image loaded and hidden again on error, so a
preview that failed and a preview that did not exist looked identical —
an empty gap under the dropdown. That is what it looked like from the
outside, and it cost a diagnosis: the endpoint renders a correct 934x525
PNG in 0.66s when called exactly as the browser calls it, cookie and
all, and nothing on the page could have told anyone that.
The first view of a theme is a pptx render, a Gotenberg round trip and a
pdftoppm — a second or so before anything appears, and then cached on
disk. A second of silence reads as a feature that is not there, so it
now says it is drawing, and on failure says the look itself is
unaffected, which is true: the preview is a picture of the export, not
the thing that produces it.
img.src is assigned after both handlers. A cached preview can complete
before a handler attached below the assignment exists, which would leave
the box stuck on "Drawing...".
describeTheme already ends by calling it, so the call sitting beside it
fired a second identical request on every load.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Dv6sqaY6Vq3ChZHMem3cnU
