Merge from origin/emacs-26

Merge from origin/emacs-26

4122d54 Fix updating scrollbar sizes when scaling is in effect
21a212f Collect GnuTLS extensions and use them to set %DUMBFW if supp...
936136e * test/lisp/emacs-lisp/derived-tests.el: Fix copy&paste lefto...
07b7fb9 * lisp/subr.el (delayed-after-hook-functions): Rename from .....
a5b0a4e * lisp/net/shr.el (shr-string-pixel-width): Return pixel-widt...
c51e797 python.el doc fixes
c62ced5 Make 'mouse-drag-and-drop-region' more robust and customizable