### Description For a `splom` trace whose `marker.colorbar` exists but has **no explicit `thickness`** (plotly.js defaults it), applying `marker.colorbar.thicknessmode='pixels'` afterwards with `Plotly.restyle` **collapses the whole splom layout**: - plotly.js silently writes `thickness: 16290` back into the user-visible trace (`gd.data[1].marker.colorbar.thickness`), - the colorbar then occupies ~16k px and the splom panels shrink from ~125 px to ~15 px per axis. Setting the *exact same value* at `newPlot` time (Path A in the repro) renders fine. Reproduced on every version I could test: **2.35.2, 3.5.0, 3.5.1, and current 3.8.2**. ### Steps to reproduce Save the following as `index.html` and open it in a browser (uses `https://cdn.plot.ly/plotly-3.5.0.min.js`; the bundled copy from any release works too). The page renders both paths, then prints the colorbar object and the panel width under each plot. ```html plotly.js splom + marker.colorbar restyle bug

splom layout collapses when `marker.colorbar.thicknessmode='pixels'` is applied with `Plotly.restyle`

Same data + layout, same final marker.colorbar value. Difference is only when it is applied:

Path A (normal): marker.colorbar = {thicknessmode: 'pixels'} is in the figure at newPlot time.
Path B (collapsed): figure starts with marker.colorbar = {} (no explicit thickness), then Plotly.restyle(gd, {'marker.colorbar.thicknessmode':'pixels'}, [1]).

After Path B, plotly.js writes thickness: 16290 back into the trace's colorbar and the splom panels collapse from ~125px to ~15px per axis.

Path A — set at newPlot time (normal)

…

Path B — set via Plotly.restyle (collapsed)

…

``` ### Actual vs expected (measured with plotly 3.8.2, headless Chrome) | | Path A (set at `newPlot`) | Path B (set via `restyle`) | |---|---|---| | `trace[1].marker.colorbar` after render | `{"thicknessmode":"pixels"}` | `{"thickness":16290,"thicknessmode":"pixels"}` | | panel width (`_fullLayout.xaxis._length`) | 125 px | **15 px** | | rendered colorbar width | 71 px | **16331 px** | Expected: Path B behaves like Path A — thickness defaulted to ~30 px for `pixels` mode, panels stay 125 px. (Note: with random synthetic splom data the exact numbers differ — e.g. `thickness:17250`, `axis len ~15` — but the collapse reproduces identically.) ### Analysis `colorbar.thickness` is size-dependent when left at its default: in `fraction` mode the default is ~`30 / plotHeight` (a small fraction), converted to pixels only at calc time. When `Plotly.restyle` flips `thicknessmode` to `pixels` on a colorbar that still holds the fraction-mode default, the default is apparently **not re-coerced for the new mode** — the old tiny fraction survives, is interpreted as raw pixels on the next colorbar calc, and the result (~16k px) is also written back into `gd.data` (user-visible trace), not just `_fullLayout`. Evidence that fits this reading: 1. Restyling `thickness` **together with** `thicknessmode` (`{'marker.colorbar.thickness': 20, 'marker.colorbar.thicknessmode': 'pixels'}`) works fine — an explicit value bypasses the buggy default path. 2. After Path B, the wrong value is observable on the *user data object* (`gd.data[1].marker.colorbar.thickness === 16290`), so a defaults-coercion result leaks back into `gd.data`. First reported at [plotly/plotly.py#5615](https://github.com/plotly/plotly.py/issues/5615); the plotly.py maintainers asked for a pure-JS reproduction here. ### Workaround Include an explicit `thickness` in the same restyle call: ```js Plotly.restyle(gd, { 'marker.colorbar.thicknessmode': 'pixels', 'marker.colorbar.thickness': 20 }, [1]); ```

splom layout collapses when marker.colorbar.thicknessmode='pixels' is applied with Plotly.restyle

Path A — set at newPlot time (normal)

Path B — set via Plotly.restyle (collapsed)

splom layout collapses when `marker.colorbar.thicknessmode='pixels'` is applied with `Plotly.restyle`

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