Document a GDK misdesign interfering with touch screens and menus

* etc/PROBLEMS (Runtime problems specific to PGTK): Document a
misdesign preventing menus from being displayed in response to
touch screen events.

(cherry picked from commit 50c4feb13246f1f82efd761dd13d31d5474cffa0)

diff --git a/etc/PROBLEMS b/etc/PROBLEMS
index da861ebe6e782b7a6da197461dcec670a4c687a6..085ed4d05326a4c42870c9329feb2934da3ba596 100644 (file)
--- a/etc/PROBLEMS
+++ b/etc/PROBLEMS
@@ -3512,6 +3512,22 @@ Compose key to stop working.
 On X Windows, users should not use Emacs configured with PGTK, since
 this and many other problems do not exist on the regular X builds.
 
+** On occasion, menus cannot be activated by touch screen events.
+
+Menus might not be displayed after a mode-line item, or another command
+that produces a menu, is activated by touch screen events.  This is the
+product of several bugs or misdesigns in the GDK Wayland backend which
+cannot be circumvented by Emacs.  Specifically, Wayland denies clients
+that cannot provide a sufficiently recent event serial number the
+ability to display pop-up windows, but GDK only considers the serial
+numbers of touch screen events when their touch sequences remain active,
+and, as touch screen activity is not registered by Emacs until after the
+toolkit's delivery of touch sequences completes, by the time
+`x-popup-menu' is called, the only eligible event serials are those of
+the last keyboard event and the last mouse event, which will not suffice
+if Wayland has received pop-up or drag-and-drop requests from other
+clients since their generation.
+
 * Runtime problems specific to Android
 
 ** Text displayed in the default monospace font looks horrible.