Three agent rounds chased a caption offset that a 1080x1920 constant already answered

Vertical correct and horizontal wrong came out of the same code doing the same arithmetic on a different axis ^[5]. "Maths errors don't pick a favourite axis," the author wrote ^[5]. So the two axes were being fed different numbers, and by his account each dimension collected its own rounding on the way in ^[12].

Both ends of the ratio were measured ^[6]. The preview number came from asking the browser how big the element had ended up, and what the browser hands back carries an undocumented rounding decision on top of sub-pixel layout and device pixel ratio ^[6][8]. The render number came from probing the encoded video file ^[6]. That file was a scaled-down proxy built for the preview, so the probe returned the proxy's dimensions plus whatever the encoder decided about even numbers ^[7].

The number the encoder was actually given sat in the repo, seven lines into internal/reel/timing.go, as Width = 1080, Height = 1920, FPS = 30 ^[9]. The block is commented for parity with gen-reel.py, so the same geometry lives in a Go constant and a Python script ^[10]. The code working out how big the render would be never consulted the values the team defined and passed to the encoder ^[11].

Read the constant and the ratio has one measured input left instead of two ^[20]. The one that remains is the editor element, and it has to be measured, because that is the surface the user drags on. 1080 by 1920 is 9:16 ^[19]. When the editor panel holds that aspect, I would expect a single scale factor to cover both axes. That removes the failure mode where one axis lands and the other misses.

This is one developer's post about one feature, so treat the three failed rounds as an account rather than a measurement ^[2]. For the lesson to carry into another codebase, an authoritative constant has to exist somewhere a reviewer can point at. A pipeline that takes frame size from a CLI flag or a job record has no line to read, and the contract then has to reach the model from whichever record owns it.

The author had built the same thing by hand years earlier at an agency, in JavaScript after refusing Flash, using absolute positioning and a scaling ratio, and it took a couple of days ^[15]. He had known the technique for close to twenty years ^[16]. Familiarity is why this one went out without a spec. "Laziness, mostly, and I'll own that one," he wrote ^[17], and says the handover was a list of requirements, full stop ^[18]. The rule he draws from it: "when a value is defined by your own code, don't go re-deriving it by measuring something downstream... read it" ^[13]. He reports the fix has held since ^[14].