Development/HiDpi

Workitems for HiDPI
As of LibreOffice 4.2.3, Windows and Linux have good HiDPI support, although bitmaps are scaled. Here's a writeup of the initial work. Note: simple scaling is generally best to make icons not blurry

OS / HiDPI detection issues
Gnome: Since version 3.10, Gnome auto-detects screen resolution and sets a 192 DPI value on HiDPI monitors. You can verify this by running "xrdb -query". Therefore, LibreOffice looks good out of the box.

KDE: In 4.x system settings, under Application Appearance - Fonts, you can set the "Force fonts DPI" to 192. When this is set, the LibreOffice HiDPI mode kicks in.

Xfce: It is possible to manually set the DPI to 192 as well.

Unity: System settings → Displays and using the slider "Scale for menu and title bars", set the value to 2. ("xrdb -query" returns 192 dpi after setting the scale factor 2, and the scale factor may be 2 by default for HIDPI screens in the future.)

Mac: Apple does auto-doubling until you turn off compatibility mode which causes some small side effects Norbert is looking into it but is still in the investigation phase.

Status
A first round of fixes was released in 4.2.3, and more have gone into subsequent releases: https://cgit.freedesktop.org/libreoffice/core/log/?qt=grep&q=hidpi

Toolbar
Remaining
 * Drawing object properties line style preview too thin
 * The circles which denote the left edge of a toolbar are hard to see.
 * The form navigator toolbar record selector hard-codes size

Sidebar
Remaining
 * Paragraph spacing bitmaps
 * Selected valueset items
 * Calc border style dropdown

General UI
Remaining
 * splash screen is too small (possible Hard Hack, see splashx.c)
 * hyperlink dialog box
 * multiple monitors with different resolution untested
 * Tools - Options Track Changes settings need a bit of work
 * Tools - Customize Dialog menus / toolbars
 * Insert - special characters
 * Gallery bitmaps too small?
 * Incomplete list...

Non-HiDPI specific but more noticeable

 * Format - Paragraph, etc. border default of .05 pt too small
 * Format - Paragraph, etc. shadow default of .07" too large
 * Insert Bookmark dialog box too small
 * Tab stops of .49" seem off by 1.