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desktop: work around client-side rounding issues at right/bottom pixel

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This also avoids a similar server-side rounding issue with some
combinations of wlroots and libwayland versions. See:

- https://gitlab.freedesktop.org/wayland/wayland/-/issues/555
- https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/5159

Tested with qmpanel (similar to lxqt-panel) at 1x and 2x output scale.
Does not help if scaling is done client-side, e.g. QT_SCALE_FACTOR=2.

Fixes: #2379
Fixes: #3099
This commit is contained in:
John Lindgren 2025-10-19 22:15:37 -04:00 • committed by Johan Malm
parent 1e406dd649
commit 28cd504067
1 changed files with 35 additions and 0 deletions
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35
src/desktop.c
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@ -242,6 +242,39 @@ desktop_update_top_layer_visibility(struct server *server)
}
}
/*
* Work around rounding issues in some clients (notably Qt apps) where
* cursor coordinates in the rightmost or bottom pixel are incorrectly
* rounded up, putting them outside the surface bounds. The effect is
* especially noticeable in right/bottom desktop panels, since driving
* the cursor to the edge of the screen no longer works.
*
* Under X11, such rounding issues went unnoticed since cursor positions
* were always integers (i.e. whole pixel boundaries) anyway. Until more
* clients/toolkits are fractional-pixel clean, limit surface cursor
* coordinates to (w - 1, h - 1) as a workaround.
*/
static void
avoid_edge_rounding_issues(struct cursor_context *ctx)
{
if (!ctx->surface) {
return;
}
int w = ctx->surface->current.width;
int h = ctx->surface->current.height;
/*
* The cursor isn't expected to be outside the surface bounds
* here, but check (sx < w, sy < h) just in case.
*/
if (ctx->sx > w - 1 && ctx->sx < w) {
ctx->sx = w - 1;
}
if (ctx->sy > h - 1 && ctx->sy < h) {
ctx->sy = h - 1;
}
}
/* TODO: make this less big and scary */
struct cursor_context
get_cursor_context(struct server *server)
@ -269,6 +302,8 @@ get_cursor_context(struct server *server)
ret.node = node;
ret.surface = lab_wlr_surface_from_node(node);
avoid_edge_rounding_issues(&ret);
#if HAVE_XWAYLAND
/* TODO: attach LAB_NODE_UNMANAGED node-descriptor to unmanaged surfaces */
if (node->type == WLR_SCENE_NODE_BUFFER) {