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But the mainmenu is static and doesn't allow customizing the links according to the current page context (i.e. current_checkin).

This does not answer your question (which i don't have an answer for), but mainmenu is not quite static: it's a th1 list. You can manipulate it as much as you like before rendering it (which each skin does separately):

[stephan@nuc:~/f/fossil/skins]$ grep -w mainmenu */head*.txt
ardoise/header.txt:    <nav class="mainmenu" title="Main Menu">
ardoise/header.txt:          foreach {name url expr class} $mainmenu {
black_and_white/header.txt:<nav class="mainmenu" title="Main Menu">
black_and_white/header.txt:    foreach {name url expr class} $mainmenu {
blitz/header.txt:    <nav class="mainmenu" title="Main Menu">
blitz/header.txt:          foreach {name url expr class} $mainmenu {
darkmode/header.txt:<nav class="mainmenu" title="Main Menu">
darkmode/header.txt:    foreach {name url expr class} $mainmenu {
default/header.txt:<nav class="mainmenu" title="Main Menu">
default/header.txt:    foreach {name url expr class} $mainmenu {
eagle/header.txt:<nav class="mainmenu" title="Main Menu">
eagle/header.txt:    foreach {name url expr class} $mainmenu {
etienne/header.txt:<nav class="mainmenu" title="Main Menu">
etienne/header.txt:    foreach {name url expr class} $mainmenu {
khaki/header.txt:<nav class="mainmenu" title="Main Menu">
khaki/header.txt:    foreach {name url expr class} $mainmenu {
original/header.txt:<nav class="mainmenu" title="Main Menu">
original/header.txt:    foreach {name url expr class} $mainmenu {
plain_gray/header.txt:<nav class="mainmenu" title="Main Menu">
plain_gray/header.txt:    foreach {name url expr class} $mainmenu {
xekri/header.txt:<nav class="mainmenu" title="Main Menu">
xekri/header.txt:    foreach {name url expr class} $mainmenu {

Hi Stephen:

But the mainmenu is static and doesn't allow customizing the links according to the current page context

This does not answer your question (which i don't have an answer for), but mainmenu is not quite static: it's a th1 list. You can manipulate it as much as you like before rendering it

I guess I should have said the elements in the mainmenu list are static. I can't really manipulate it but I can create a new mainmenu list with a different URL for the File link.

TH1 only supports lappend for changing a list. TCL has lreplace or lset that can be used with lsearch to replace the url parameter by searching $mainmenu for File then using lset/lreplace on the returned lsearch index + 1.

The following script creates an lset proc and uses it to replace the url. The lset only accepts one index and not an index list (like TCL's lset) to handle nested list structures.

proc lset {listName index replacement} {
    upvar 1 $listName theList
    set current_index 0
    set newmenu {}
    foreach item $theList {
    if {$current_index == $index} {
        lappend newmenu $replacement
    } else {
        lappend newmenu $item
    }
    set current_index [expr $current_index+1]
    }
    return $newmenu
}


set mainmenu {Home      /home        *              {}
Timeline  /timeline    {o r j}        {}
Files     /dir?ci=tip  oh             desktoponly
Branches  /brlist      o              wideonly
Tags      /taglist     o              wideonly
Forum     /forum       {@2 3 4 5 6}   wideonly
Chat      /chat        C              wideonly
Tickets   /ticket      r              wideonly
Wiki      /wiki        j              wideonly
Admin     /setup       {a s}          desktoponly
Logout    /logout      L              wideonly
Login     /login       !L             wideonly}

# print starting case
# Its formatting is much better then the list
# I construct. Not sure why.
puts "$mainmenu\n"

# Find the Timeline
puts "\n[lindex $mainmenu 4]\n\n"

# find the File entry
set file_loc [lsearch $mainmenu "Files"]

if {[expr "$file_loc % 4"] != 0} {
    error "ERROR: label found in the wrong position"
    # not sure how I can use lsearch to find the next match
    # in the list past the one that was returned. TCL's -start
    # or -all flags don't seem to be supported.
}

# do the switch
set mainmenu [lset mainmenu [expr $file_loc+1] "/dir?ci=deadbeef"]

# print post switch case.
# without [list ...] puts doesn't put {}'s around lists printing
# "{a s}" as "a s". I need to use foreach to insert line breaks.
foreach {name url perm when} $mainmenu {
    puts "[list $name $url $perm $when]\n"
}

# make sure the Timeline is at the same location.
puts "\n[lindex $mainmenu 4]"
puts ""

producing:

Home      /home        *              {}
Timeline  /timeline    {o r j}        {}
Files     /dir?ci=tip  oh             desktoponly
Branches  /brlist      o              wideonly
Tags      /taglist     o              wideonly
Forum     /forum       {@2 3 4 5 6}   wideonly
Chat      /chat        C              wideonly
Tickets   /ticket      r              wideonly
Wiki      /wiki        j              wideonly
Admin     /setup       {a s}          desktoponly
Logout    /logout      L              wideonly
Login     /login       !L             wideonly

Timeline

Home /home * {}
Timeline /timeline {o r j} {}
Files /dir?ci=deadbeef oh desktoponly
Branches /brlist o wideonly
Tags /taglist o wideonly
Forum /forum {@2 3 4 5 6} wideonly
Chat /chat C wideonly
Tickets /ticket r wideonly
Wiki /wiki j wideonly
Admin /setup {a s} desktoponly
Logout /logout L wideonly
Login /login !L wideonly

Timeline

TH1 only supports lappend for changing a list. ...

True, you're very limited in the list operations¹, But you could add a placeholder entry such as:

BLAH x oh {}

Pass all other entries through as-is and replace the 2nd element of that one with /dir?ci=....

Not that i'm recommending that - th1 is pretty limited, so there may well be some frustration or dead-end limitations involved.

Fossil has a rarely-used build-time configuration flag, --with-tcl, which enables th1 to embed tcl code, but my experience with that option is 0 so i won't do more than point it out as a possibility for custom fossil builds.

That still doesn't(?) solve the taint problem, though, for which i've got no recommendations (for lack of know-how regarding that recently-added capability).

Fossil has a rarely-used build-time configuration flag, --with-tcl, which enables th1 to embed tcl code, but my experience with that option is 0 so i won't do more than point it out as a possibility for custom fossil builds.

I build --with-tcl on all my systems. However I can't count on a user who clones the repo having a tcl enabled fossil build, hence limiting my solution to TH1.

I put a real entry for the BLAH url as a fallback. If somebody updates the mainmenu list and changes BLAH at least they will have the URL setting they expect. Only if BLAH is "Files" (not "files", "directory" etc...) will my example change it to the checkin specific link. This way they can reorder the main menu and still have it work.

httpize -- Is this the correct fix?

Probably, yes. I've come across this when updating the Custom Skins document with information about tainted strings.

I've now also added the current_checkin variable to the docs, and from the source code it's clear that it's derived from query parameters and therefore unsafe (or "tainted").