Redmine

1. Performance Issue Root Cause and Fix (Desktop vs Mobile Breakpoint)

We investigated a severe input latency problem on Redmine issue pages with many form fields.

In desktop layout (≥900px width), typing in input fields caused extreme delays (2–6 seconds per keystroke).
In responsive/mobile layout (<900px), the same page performed normally (~10–30ms latency).

1. Key Finding

The performance difference was not caused by the menu itself or by the number of event handlers.
Instead, the root cause was a layout/positioning behavior related to the main page container:

- Desktop layout: `#wrapper` uses the default `position: static`
- Mobile layout: `#wrapper` becomes `position: relative`

Adding the following CSS rule immediately eliminated the input lag:

#wrapper {
  position: relative;
}

1. Why This Fix Works

Although `position: relative` does not visually move the wrapper, it changes browser layout behavior in an important way:

- It creates a stable containing block for absolutely positioned UI elements (dropdowns, autocompletes, datepickers, overlays).
- Without a positioned ancestor, such elements may be positioned relative to the document/body, which can trigger expensive global reflow/repaint operations.
- On large pages with many inputs, this can cause layout thrashing during typing (forced synchronous reflows on each key event).

Once `#wrapper` becomes a positioned ancestor, layout calculations become more localized and much cheaper, restoring normal responsiveness.

1. Evidence

A breakpoint diff at 900px → 899px showed:

- `#wrapper` changes from `position: static` → `position: relative`
- Menus/sidebar are moved into an offscreen flyout structure
- Input latency drops from seconds to milliseconds

1. Recommendation

Consider setting `#wrapper { position: relative; }` (or applying it specifically on issue pages) as a default, since it prevents catastrophic reflow behavior on large forms.

This appears to be a browser layout containment issue triggered by missing positioned ancestors in desktop mode.

Thanks for the report. To reproduce this, could you share roughly how many full-width long text fields are on the issue edit form?

21 to be exact but I got a feeling it's starting to be noticeable with 8-10

Yes that would probably be a good thing. My plugins (Additionals being a candidate) adds GUI elements which might be what's triggering this issue. Thank you for looking into this.