Thanks for filing this Martijn, took a look at this today and it should be possible to fix. 

[self.tableView rectForHeaderInSection:section] is not recognizing a floating header and I'm not sure why. I'll try to see whats wrong with this approach or try a new one.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e50f3dd4d1d6a59d8e0006bf66399950f3a5342b

commit e50f3dd4d1d6a59d8e0006bf66399950f3a5342b
Author: sczs <sczs@chromium.org>
Date: Wed Jun 27 01:50:24 2018

[ios] Adds gesture recognizer directly on RecentTabs Headers

Bug:  851395 
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I2199f83c410d9e42fa9e8c990c9b527ff9608156
Reviewed-on: https://chromium-review.googlesource.com/1115507
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570625}
[modify] https://crrev.com/e50f3dd4d1d6a59d8e0006bf66399950f3a5342b/ios/chrome/browser/ui/recent_tabs/recent_tabs_table_view_controller.mm