Symptom - when Object Snaps (Osnaps) are on and you hover near a snap point, the display should indicate which snap is active (with a word like End or Cen in a little box near the cursor). In certain installations, this tooltip is distorted or unreadable. Typically people report seeing a square box with a fuzzy or garbled display of pixels inside.
Fix - Right click on the desktop, choose Properties, go to the Settings tab, click the Advanced button, go to the Troubleshoot tab and move the little slider one or two clicks to the left. The problem should go away.
This has to do with the graphics card, drivers used, and your OpenGL settings. It generally only happens when using hardware accelerated OpenGL, and is most often reported in conjunction with ATI graphics cards and drivers. Moving the slider to the left one or two clicks removes the mouse cursor acceleration (without affecting the rest of the hardware acceleration), which seems to fix the problem in most cases.
Note: There may be some cases where the above method does not work or will not be possible, see the bottom of the page for another way to fix this (Rhino 4 only)
removes the mouse cursor acceleration,
but not the rest of the hardware acceleration.
In some cases, you are not allowed to change the slider indicated above - it will be grayed out - for example a low-level user on a network might not have privileges to change display settings.
If this is the case, you should try the following:
Go into Rhino Options > Modeling Aids. In the Dynamic Object Snap section, check “None”
Before exiting Options, go to the Cursor ToolTips section (a sub-category of the modeling aids section) and check the top box “Enable cursor tooltips”
Afterwards, check “Osnap”, then OK to exit Options.
Hopefully you should now have a working osnap onscreen display. The cursor tooltips use a different display method, and are not affected by the same OpenGL problems as outlined in the first section. If there are still problems, report these to the Rhino newsgroup or tech support.