https://bugs.kde.org/show_bug.cgi?id=528228
Bug ID: 528228
Summary: Panel hangs in childAtCoordinates() while something is
dragged over it when the layout spacing is 0
Classification: Plasma
Product: plasmashell
Version First 6.8.2
Reported In:
Platform: NixOS
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: Panel
Assignee: [email protected]
Reporter: [email protected]
Target Milestone: 1.0
plasmashell's main thread hangs at full CPU while something is dragged over a
horizontal panel whose layout has a rowSpacing of 0. On 2026-10-28 it answered
no D-Bus call for 25 s, and `systemctl --user restart plasma-plasmashell` ended
it. The spacing is 0 on my panel because the third-party Panel Colorizer widget
sets the panel's widget spacing to 0; the stock panel uses
Kirigami.Units.smallSpacing.
In containments/panel/LayoutManager.js, childAtCoordinates() steps x back by
layout.rowSpacing (y by columnSpacing on a vertical panel) until
layout.childAt() finds a widget. With a spacing of 0, x never changes, so the
loop never ends when no widget is under the start point. The function is the
same on master.
To check the function on its own, I ran master's LayoutManager.js in Node 24
with a mocked layout that has one widget from x 0 to 200, and asked for the
child at x 300:
```
import { readFileSync } from "node:fs";
const src = readFileSync(process.argv[2], "utf8");
for (const spacing of [4, 0]) {
let calls = 0;
const layout = {
width: 500, height: 30, rowSpacing: spacing, columnSpacing: spacing,
childAt(x, y) {
if (++calls > 1e7) throw new Error(`still looping after 1e7 childAt
calls, x=${x}`);
return x < 200 ? { index: 0, x: 0, width: 200 } : null;
},
};
const root = { isHorizontal: true };
const childAtCoordinates = new Function("root", "layout", `${src}; return
childAtCoordinates;`)(root, layout);
try {
const child = childAtCoordinates(300, 10);
console.log(`rowSpacing ${spacing}: returned the widget with index
${child.index} after ${calls} childAt calls`);
} catch (e) {
console.log(`rowSpacing ${spacing}: ${e.message}`);
}
}
```
It printed:
```
rowSpacing 4: returned the widget with index 0 after 27 childAt calls
rowSpacing 0: still looping after 1e7 childAt calls, x=300
```
Should the search step at least 1 px, or is a spacing of 0 not supported on the
panel layout? I patch it locally to step by max(1, spacing) and to start no
further out than the end of the last widget.
NixOS 26.11, Plasma 6.8.2, Frameworks 6.31.0, Qt 6.12.0, Wayland.
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