Transient surfaces must stay on the current display
Aliases: popover across monitors · menu off-screen · bezel clipping
What it is
Menus, tooltips, dialogs, color pickers, and autocomplete lists must paint inside the display that owns them. Positioning against the whole virtual desktop lets a surface ride the bezel between two screens, or jump entirely onto the neighbor—unreadable on one side, “it never opened” on the other.
The current display is the one that holds the pointer, or the one that holds the anchor window, not “whichever has the larger coordinates.”
Why it happens
Screens are adjacent in coordinates and not adjacent in vision. Between them sits a bezel, a gap, sometimes a band that no panel lights. Baudisch and colleagues treated that seam as a pointing obstacle: the cursor can cross it; the eyes cannot read two severed halves as one object. A menu split left and right is cropped at the bezel, and its hit region breaks in two. A dialog centered in global desktop coordinates may appear in the middle of the neighbor while the operator stares at the original window and decides the command was swallowed.
Transient surfaces are short-lived and attention-hungry; they tolerate less error than the main window. A main window that crosses screens still has a title bar you can drag; a tooltip that flashes on the wrong panel is equivalent to never opening. Placement should treat the visible rectangle of the owning display as a cage: prefer the anchor, flip if needed, then shrink or scroll inside this screen—never borrow the overflow from a neighbor.
Studying it
On a dual-display desk, fire menus, tooltips, and modal dialogs, and record the intersection of each popup rectangle with each screen’s visible rectangle. Compare “position in global desktop coordinates” against “clip to the owning display”: count bezels straddled, wholesale jumps to the neighbor, and times the operator turns back to hunt on the original screen. Baudisch’s Mouse Ether and Drag-and-Pop treat the bezel as a discontinuity; they sit on the same seam.
Independent variables: distance from the anchor to the screen edge, relative placement of the two displays (left–right / above–below), placement strategy. Dependent variables: fraction of the popup that intersects its owning display, straddle count, time to notice the popup, times it is taken as unopened.
Testing only a left–right pair misses vertical straddles on stacked displays. A laptop lid closed against an external can make “current display” briefly a panel that is no longer lit.
Where it stops holding
Exclusive fullscreen (slideshow, game) already cages transients to that canvas; the problem becomes “don’t paint off-screen,” not “don’t paint on the neighbor.” A single display never meets this. In mirror mode the same picture appears on both panels, so a jump is still visible, but which panel receives the click can still scramble. A screen magnifier that pipes pixels onto another display should take “current” as the panel being looked at, not only the window’s coordinates.
Applying it
- Compute popup position against the owning display’s visible rectangle only: prefer the anchor, then flip, shift, or shrink inside this screen; never cross the seam.
- Center modal dialogs on the display that owns the parent window, not on the geometric centre of the virtual desktop.
- Verify by dragging the window to all four edges of every display and opening menus, tooltips, dialogs, and a date picker. Any surface that straddles the bezel, appears on a neighbor, or leaves the original screen unchanged has failed.