[3.0] Link the first page of a topic on the message index

[albertlast]

Description

A topic that runs to more than one page gets a little Pages 1 2 3 index beside it on the message index. Page 1 comes out as plain text rather than a link, so the first page of every multi-page topic is the one page you cannot click through to.

PageIndex has a branch for exactly this case. A caller with no current page to mark passes a negative start, fixStart() records that in $start_invalid, and pageRange() then renders even the "current" page as a link:

// If start was invalid, show page number as a link to the proper start value.
if ($this->start_invalid) {
	$page_range .= sprintf($this->base_link, $this->start, $this->current_page_num);
}

That branch has never run. __toString() calls fixStart() a second time, reasonably enough — $this->start is public and may have been changed since construction:

// Ensure $this->start is still good, just in case someone changed it.
$this->start = $this->fixStart($this->start);

But fixStart() does double duty: it clamps the value and records whether what it was given was negative. By the time __toString() runs, the constructor has already clamped the caller's -1 to 0, so the second call looks at 0, decides the start is perfectly valid, and overwrites the verdict recorded the first time. $start_invalid is therefore false every time it is read.

The fix keeps the earlier verdict and lets the second look only add to it, so a start that has gone negative since construction still counts.

Effect

MessageIndex::buildTopicContext() is the one caller that passes -1 deliberately. On a topic spanning three pages:

	markup for page 1
before	<span class="current_page">1</span>
after	<a class="nav_page" href="…?topic=1.0">1</a>

Pages 2 and 3 are unchanged, as is the Pages label.

The board's own page index is unchanged — there the start really is 0 and page 1 really is the current page:

page index	before	after
topic row (#pages1)	current_page	link
board's own (.pagelinks)	current_page	current_page
board with a negative start in the URL	current_page	current_page

That last row is unchanged because the board URL parser clamps the start well before PageIndex sees it, so this stays confined to callers that pass a negative start on purpose.

Found while triaging the message index for #7933; it changes no template, so it goes on its own.

Issues References (Fixes|Related|Closes)

Related: #7933