xdg-shell: on xdg_popup.grab serial validation

xdg_popup.grab:

This request must be used in response to some sort of user action like a button press, key press, or touch down event. The serial number of the event should be passed as 'serial'.

However, a grab may also be started in response to a pointer motion when the user moves the cursor to a popup menu item which opens another popup. Therefore, clients try to use what serial they think fits the most.

Current behavior

Note that the following analysis might be not completely correct.

GTK

GTK uses

• last wl_keyboard.key serial, regardless of the key state,
• last wl_pointer.button press serial,
• last zwp_tablet_tool_v2.{down,button} serial for one of the tools, regardless of the button state,
• last wl_touch.down serial for each valid touch point, or
• last wl_touch.down serial received.

whichever is newer.

Qt

Qt uses

• last wl_keyboard.key press serial,
• last wl_pointer.button press serial,
• last zwp_tablet_tool_v2.down serial for one of the tools, or
• last wl_touch.down serial,

whichever is newer.

Note that tablet tool button serials are ignored.

Mutter

Mutter expects the serial to be

• one of the wl_keyboard.key press serials matching currently held keys, or
• last wl_keyboard.key release serial if it's the latest key event,
• last wl_pointer.button press serial that was corresponding to an implicit grab, or
• last wl_touch.down serial for each valid touch point, or
• last wl_touch.down serial sent, or
• last zwp_tablet_tool_v2.{down,button} serial for one of the tools, regardless of the button state.

The set of currently held keys is reset on keyboard focus change.

The implicit grab mentioned is not required to be active by the time of serial validation.

Weston

Weston expects the serial to be

• last wl_keyboard.key serial, regardless of the key state, or
• last wl_pointer.button press serial that was corresponding to an implicit grab, or
• last wl_touch.down serial that was corresponding to an implicit grab, or
• last zwp_tablet_tool_v2.{down,button} serial for one of the tools, regardless of the button state.

The implicit grabs mentioned are not required to be active by the time of serial validation.

Problems

A Qt program (Dolphin) in Mutter/Weston

Astute readers may have noticed that opening a Qt popup in Mutter/Weston with a key press (F10) and trying to open a nested popup by pointer motion will fail:

Qt attempts to use the last key press serial for it, which is already dropped.

Notably, if F10 is kept pressed, then in Weston (but not in Mutter) the serial is still considered valid for a popup grab.

A GTK program in Mutter

Opening a menu popup with a key press (Alt+F) in Mutter and trying open a different menu popup by pointer motion will fail. This happens to due Mutter clearing the pressed key serials set on keyboard focus change. For the same reason, opening a menu popup with a pointer button press, pressing and releasing a key, and trying open a different menu popup by pointer motion fails as well.

Thoughts

The nested popup problem can be resolved by simply™ ignoring xdg_popup.grab requests for popups if one of their ancestors is a grabbing popup. As per request description, a grab ensures that 1) keyboard focus stays on the topmost grabbing popup, and 2) pointer/touch events are only delivered to the popup client; "toplevel → grabbing popup → grabbing popup" and "toplevel → grabbing popup → normal popup" chains then result in identical behavior.

The "I had a grabbing (menu) popup open, and now I need to close it and open a new one" problem might be solvable by storing the serial that allowed the previous grab (maybe for a limited time) and using it for validation too.

However, this raises a question: is there any benefit to having this serial validation logic in the protocol in the first place? To me, it feels like a "is the toplevel this popup is for focused?" check would be enough to prevent focus stealing and such. Notably, the vast majority of compositors I've looked at (KWin, Jay, Hyprland, as well as all wlroots-based compositors) just ignore xdg_popup.grab serials.

Is the Qt problem a bug in Qt or in compositors' serial validation logic? It's not immediately clear to me.