Made it illegal to call Begin("") with an empty string. This somehow accidentally worked before but had various undesirable side-effect as the window would have ID zero. In particular it is causing problems in viewport/docking branches.

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.cpp
#	imgui.h

Viewport: Corrected/clarified comments. Moved RenderPlatformWindowsDefault() next to UpdatePlatformWindow(). Removed unnecessary flag check.

IO: Realigned all fields, very minor comments change. This is nearly a no-op if you don't ignore Spaces.

IO: Added BackendPlatformUserData, BackendRendererUserData, BackendLanguageUserData void* for storage use by back-ends. (#2004 + for cimgui)

Changed ImGuiCol_ChildBg to (0,0,0,0) in Dark style instead of (1,1,1,0), to match other styles. Shouldn't have any effect for the end-user.

Fixed using SetNextWindowPos() on a child window (which wasn't really documented) position the cursor as expected in the parent window, so there is no mismatch between the layout in parent and the position of the child window. Demo tweak and adding some child window stuff

Various user-facing comments

Tidying up README, moved entries to FAQ, updated screenshots, removed comma in title.

Fix merge issue on master.

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.cpp

Window, Focus, Popup: Fixed an issue where closing a popup by clicking another window with the _NoMove flag would refocus the parent window of the popup instead of the newly clicked window.

Moved setting up NavHideHighlightOneFrame from lower-level ClosePopupToLevel() to CloseCurrentPopup() with an explanation. (Followup on 68d3e13)

Docs: various updates, rewording, clarifying the purpose of a PR.

Internals: Popups: Renamed CurrentPopupStack to BeginPopupStack which is much less ambiguous.

Internals: Popups: EndMenu() calls ClosePopupToLevel(g.CurrentPopupStack.Size) which is more correct.

Internals: Popup related comments. Renamed the misleading internal ClosePopup() function. Added bool* test to BeginPopupModal in demo.

Viewport, DPI: Now using DpiScale from the ImGuiPlatformMonitor array instead of calling Platform_GetWindowDpiScale() before the platform window creation. Might even tentatively see if things work out without Platform_GetWindowDpiScale. (#1676)

Tests: Changed prototype of ImGuiTestEngineHook_ItemAdd to match functions called in same spot. Made ButtonBehavior submit fallback item info if ItemAdd() was not called (for resize grips, resize borders, scrollbar, columns, etc.)

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.h

Comments, minor tweaks.

Metrics: Fixed crash when viewports are disabled (g.MouseLastHoveredViewport is never set).

Tabs: Fixed support for drag and drop ImGuiButtonFlags_PressedOnDragDropHold. (#261) incorrectly missing from the merge from Docking branch.

Internals: Windows hidden with HiddenFramesRegular (but NOT HiddenFramesForResize) preserve their SizeContents, so restoring a auto-resize window after it's been hidden by tabs won't reset its size for a frame. Arguable. Let's see how it goes. (Followup to b48e295)

Contents size is preserved while a window collapsed. Fix auto-resizing window losing their size for one frame when uncollapsed.

Demo: Using Tabs in Style Editor and Simple Layout example. + Adding missing early out in About and Documents examples.

BeginTabBar: Fix to push the expected ID into the ID stack (instead of a hash's hash). (#261)

Demo: Fix collateral damage of 54a60aa

Demo: Fix collateral damage of 54a60aa

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.cpp
#	imgui_internal.h