E-book viewer: incorrect ToC highligting after changing font size

[etc205]

1. Open the attached file, "book.epub", in the viewer.
2. Press Ctrl+t to display the table of contents.
3. Click on the "Introduction" ToC item.
4. Press Ctrl++ three times to increase font.
5. Click on the "2.7: DRM: the bane of conversion" ToC item.
6. Press Ctrl+0 to restore the default font size. The "2.3: Conversion basics" ToC item gets incorrectly highlighted.

Calibre version: 6.25,
OS: Windows 10 Home.

[Kovid Goyal (kovid)]

Fixed in branch master. The fix will be in the next release. calibre is usually released every alternate Friday.