There is two problems here:

1) the flowbox single-click mode currently insists that the press and release happened in the same child - this can be worked around with a new activate-on-release mode

2) we need some way to handle the state between the long-press gestures on the parent flowbox and the multipress gesture on the child flowbox. I tried synthesizing a release event for the parent and a press event for the child, but that did not have the desired effect

Created attachment 364148 [details] [review]
gtkgesturemultipress: Add ::unpaired-release signal

This signal will be emitted whenever the gesture received a
button release or touch end event without a pairing button
press or touch begin. This usually happens when grabs transfer
input from one widget to another mid-press.

Created attachment 364149 [details] [review]
gtkflowbox: Activate items on ::unpaired-release

If the flowbox is single-click.

On master with these 2 patches it kinda works, crossing events aren't still properly sent to flowbox items though.

Review of attachment 364148 [details] [review]:

::: gtk/gtkgesturemultipress.c
@@ +326,3 @@
+      g_signal_emit (controller, signals[UNPAIRED_RELEASE], 0,
+                     x, y, button, sequence);
+    }

Should there be a return here, or do we still want to feed the event to the parent handler ?

@@ +422,3 @@
+   * its implicit grab.
+   *
+   * Since: 3.93.

Should be 3.94, at least thats what we've been using so far.

Review of attachment 364149 [details] [review]:

Looks good. GtkListBox should probably get the same treatment, just to stay in sync.

Attachment 364148 [details] pushed as bce9155 - gtkgesturemultipress: Add ::unpaired-release signal
Attachment 364149 [details] pushed as 0567b41 - gtkflowbox: Activate items on ::unpaired-release