Tables: do not show empty tooltip when user submits TableHeader with …

[lukaasm]

…no label to display

When building custom table header with sorting enabled:

        if ( ImGui::BeginTable( "##test", 2, ImGuiTableFlags_Sortable | ImGuiTableFlags_SizingStretchSame, { 600.0f, 300.0f } ) )
        {
            ImGui::TableSetupColumn( "Column1", ImGuiTableColumnFlags_WidthFixed, 110.0f );
            ImGui::TableSetupColumn( "Column2" );

            ImGui::TableNextRow( ImGuiTableRowFlags_Headers );
            if ( ImGui::TableSetColumnIndex( 0 ) )
            {
                ImGui::Button( ICON_FA_ARROW_DOWN_A_Z );

                ImGui::SameLine();

                const char * column_name = ImGui::TableGetColumnName( 0 );
                ImGui::AlignTextToFramePadding();
                ImGui::Text( column_name );

                ImGui::SameLine( 0.0f, 0.0f );

                //! use TableHeader here so we still retain table header highlight and context menu support
                ImGui::TableHeader( "##column" );
            }

            if ( ImGui::TableSetColumnIndex( 1 ) )
                ImGui::TableHeader( ImGui::TableGetColumnName( 1 ) );

            ImGui::TableNextRow();

            ImGui::EndTable();
        }

I am calling ImGui::TableHeader( "##column" ); so I still retain native table header highlight and context menu support.

But submitting label with no text to display, causes empty tooltip to be shown when the cell is hovered:

The issue is caused by the calculation of ellipsis_max:

    const float ellipsis_max = cell_r.Max.x - w_arrow - w_sort_text;

which goes below label_pos.x:

and makes condition of:

    const bool text_clipped = label_size.x > (ellipsis_max - label_pos.x);

wrongly positive.

This PR forces an assumption that the ellipsis position can not go below the min x position of the label.

[ocornut]

Merged as e49d31a, thank you!
FYI I cannot actually repro your issue with the repro above, but removing the { 600.0f, 300.0f } table size, resizing host window down and changing e.g. "Column2" to "##Column2" exhibit same issue.