fix: keep the unsupported-flag warning off browsers

--enable-blink-features is on the list of flags Chromium warns about, so every
browser the flag was applied to put a yellow "unsupported command-line flag"
bar above each page.

Browsers are given --enable-features=MiddleClickAutoscroll instead. It asks for
the same thing - Blink generates a feature of the same name for each of its
runtime flags - and is not on that list. That spelling only works from Chromium
124 onwards, so everything else keeps the flag that works everywhere: an
application embedding an older Chromium, Steam's CEF among them, has no such
bar to show anyway.

Helium knows the feature under a name of its own and ignores the Chromium one,
so browsers are asked for HeliumMiddleClickAutoscroll as well; autoscroll never
worked there before. A name a browser does not know is ignored, which is what
makes one list safe for all of them.

An installation set up by an earlier version is taken back and written again
once, because a file that is already marked as patched would otherwise be left
alone with the old flag in it.

src/lib/common.sh

@@ -13,11 +13,42 @@ MCA_PRETTY="Middle-Click Autoscroll"
 # The whole point of the package. Blink implements Windows-style autoscroll
 # behind a runtime flag that Chromium does not turn on for Linux, because
 # middle click is taken by primary-selection paste there.
+#
+# There are two ways to ask for that flag, and which one is right depends on
+# what is being started:
+#
+#   --enable-blink-features=MiddleClickAutoscroll is the one that works
+#   everywhere, and the only one that works in the Chromium versions embedded
+#   applications ship - Steam's CEF, an Electron a few years old, anything
+#   before Chromium 124. A browser started with it puts a yellow bar above the
+#   page saying an unsupported command line flag is in use, because Chromium
+#   keeps a list of flags worth warning about and this is on it.
+#
+#   --enable-features=MiddleClickAutoscroll is not on that list, so a browser
+#   started with it says nothing. Blink generates a feature of the same name
+#   for every one of its runtime flags, which is what makes the two the same
+#   request - but only since Chromium 124. Before that the name means nothing
+#   and is ignored without a word.
+#
+# So a browser, which is the only thing that shows the bar and the one thing
+# that keeps itself up to date, is given the quiet one; everything else keeps
+# the one that always works.
 MCA_FLAG="--enable-blink-features=MiddleClickAutoscroll"
 
-# The Blink feature name on its own, for merging into an --enable-blink-features
-# list that an app (or the user) already carries.
+# Helium ships the feature under its own name and does not answer to the
+# Chromium one, so browsers are asked for both. A name a browser does not know
+# is ignored, which is what makes one list safe to hand to all of them.
+MCA_BROWSER_FLAG="--enable-features=MiddleClickAutoscroll,HeliumMiddleClickAutoscroll"
+
+# The feature names on their own, for merging into a list that an application
+# (or the user) already carries, and for taking them back out again.
 MCA_FEATURE="MiddleClickAutoscroll"
+MCA_BROWSER_FEATURES="MiddleClickAutoscroll,HeliumMiddleClickAutoscroll"
+
+# Bumped when the flags above change. An installation that was set up by a
+# version with a different answer is taken back and done again once, because a
+# file that is already patched is otherwise left alone. See mca_apply.
+MCA_FLAG_SCHEME=2
 
 MCA_LIBDIR="${MCA_LIBDIR:-@LIBDIR@}"
 MCA_LOCALEDIR="${MCA_LOCALEDIR:-@LOCALEDIR@}"