Commit message:

The current text layer approach based on absolutely positioned <span> elements by default causes flickering with text selection, and we have browser-specific workarounds to solve that.

In Chrome, the workaround involves moving the .endOfContent element to right after the last element that contains some selected content. This works well in simple PDFs, but breaks when we have span.markedContent elements. Given a text layer structure like the following, rendered as four consecutive lines:

<span class="markedContent">
  <br>
  <span>development enter the construction phase (estimated at around</span>
</span>
<span class="markedContent">
  <br>
  <span>300 MEUR).</span>
</span>
<span class="markedContent">
  <br>
  <span>Kreate's EBITA increased to 2.8 MEUR (Q4'23: 2.7 MEUR) and the</span>
</span>
<span class="markedContent">
  <br>
  <span>margin rose to 3.7% (Q4'23: 3.4%). However, profitability was</span>
</span>

when starting to select from inside the first line and dragging down to the empty space after the second line, Chrome will anchor the selection at the beginning of either the <br> or the <span> inside the last .markedContent, depending on whether the selection is in "per-character mode" (i.e. click and drag) or "per-word mode" (i.e. double click and drag). This causes us to insert the .endOfContent element in the wrong place (one element too far), which causes one more line to be selected, which triggers another "selecctionchange" event, which causes us to move .endOfContent again, and so on, looping until when the whole page is selected.

This commit fixes the issue by making sure that when the end of the selection range points to the begining of an element, we walk back the dom finding the first non-empty element, and attatch .endOfContent to the end of that.

Note that this PR only touches Chrome-specific code. It's stripped away in MOZCENTRAL builds, and it is behind feature detection in GENERIC builds. Firefox already passes the new tests.

The test PDF is the second page of https://www.inderes.fi/files/42dd9cb4-1021-4f9a-ae29-cfb10ee7529c. This is the video of the current broken behavior: