InputText: Fixed various display corruption related to swapping the underlying buffer while a input widget is active (both for writable and read-only paths). Often they would manifest when manipulating the scrollbar of a multi-line input text.

imgui_internal.h

@@ -572,10 +572,11 @@ struct IMGUI_API ImGuiMenuColumns
 struct IMGUI_API ImGuiInputTextState
 {
     ImGuiID                 ID;                     // widget id owning the text state
-    int                     CurLenW, CurLenA;       // we need to maintain our buffer length in both UTF-8 and wchar format.
+    int                     CurLenW, CurLenA;       // we need to maintain our buffer length in both UTF-8 and wchar format. UTF-8 len is valid even if TextA is not.
     ImVector<ImWchar>       TextW;                  // edit buffer, we need to persist but can't guarantee the persistence of the user-provided buffer. so we copy into own buffer.
     ImVector<char>          TextA;                  // temporary UTF8 buffer for callbacks and other operations. this is not updated in every code-path! size=capacity.
     ImVector<char>          InitialTextA;           // backup of end-user buffer at the time of focus (in UTF-8, unaltered)
+    bool                    TextAIsValid;           // temporary UTF8 buffer is not initially valid before we make the widget active (until then we pull the data from user argument)
     int                     BufCapacityA;           // end-user buffer capacity
     float                   ScrollX;                // horizontal scrolling/offset
     ImStb::STB_TexteditState Stb;                   // state for stb_textedit.h