This is supposed to be a fix for bug #12311 . This bug involves a recursive message loop where the application forces visibility of scrollbars for the richedit control, which causes a WM_SIZE that triggers an update of the window size and re-hiding of the scrollbar. However, just after exiting the richedit winproc, the app forces visibility again, which again triggers a WM_SIZE, and so on. Native is oblivious to the fact that an external agent is messing with the scrollbar visibility, and is therefore unaffected by recursion. This patch attempts to fix this behavior in builtin and test for it. I have tried to test it on WinXP-SP2 and Win98, but testing on other platforms is helpful too.
Changelog: * Use internal copy of scrollbar state instead of directly reading state from scrollbars, just as native does. * Tests for ability to externally modify scrollbars. * Test for behavior that caused recursion until builtin richedit was fixed.